Electrochemical sensor based on 3D-printed substrate by masked stereolithography (MSLA): a new, cheap, robust and sustainable approach for simple production of analytical platforms.

The development of miniaturized, sustainable and eco-friendly analytical sensors with low production cost is a current trend worldwide. Within this idea, this work presents  the innovative use of masked stereolithography (MSLA) 3D-printed substrates for the easy fabrication of pencil-drawn electrochemical sensors (MSLA-3D-PDE). The use of a non-toxic material such as pencil (electrodes) together with a biodegradable 3D printing resin (substrate) allowed the production of devices that are quite cheap (ca. US$ 0.11 per sensor) and with low environmental impact. Compared to paper, which is the most used substrate for manufacturing pencil-drawn electrodes, the MSLA-3D-printed substrate has the advantages of not absorbing water (hydrophobicity) or becoming crinkled and weakened when in contact with solutions. These features provide more reproducible, reliable, stable, and long-lasting sensors. The MSLA-3D-PDE, in conjunction with the custom cell developed, showed excellent robustness and electrochemical performance similar to that observed of the glassy carbon electrode, without the need of any activation procedure. The analytical applicability of this platform was explored through the quantification of omeprazole in pharmaceuticals. A limit of detection (LOD) of 0.72 µmol L-1 was achieved, with a linear range of 10 to 200 µmol L-1. Analysis of real samples provided results that were highly concordant with those obtained by UV-Vis spectrophotometry (relative error ≤ 1.50%). In addition, the greenness of this approach was evaluated and confirmed by a quantitative methodology (Eco-Scale index). Thus, the MSLA-3D-PDE appears as a new and sustainable tool with great potential of use in analytical electrochemistry.

Hand drawing of pencil electrodes on paper platforms for contactless conductivity detection of inorganic cations in human tear samples using electrophoresis chips.

This paper describes for the first time the fabrication of pencil drawn electrodes (PDE) on paper platforms for capacitively coupled contactless conductivity detection (C(4) D) on electrophoresis microchips. PDE-C(4) D devices were attached on PMMA electrophoresis chips and used for detection of K(+) and Na(+) in human tear samples. PDE-C(4) D devices were produced on office paper and chromatographic paper platforms and their performance were thoroughly investigated using a model mixture containing K(+) , Na(+) , and Li(+) . In comparison with chromatographic paper, PDE-C(4) D fabricated on office paper has exhibited better performance due to its higher electrical conductivity. Furthermore, the detector response was similar to that recorded with electrodes prepared with copper adhesive tape. The fabrication of PDE-C(4) D on office paper has offered great advantages including extremely low cost (< $ 0.004 per unit), reduced fabrication time (< 5 min), and minimal instrumentation (pencil and paper). The proposed electrodes demonstrated excellent analytical performance with good reproducibility. For an inter-PDE comparison (n = 7), the RSD values for migration time, peak area, and separation efficiency were lower than 2.5, 10.5, and 14%, respectively. The LOD's achieved for K(+) , Na(+) , and Li(+) were 4.9, 6.8, and 9.0 µM, respectively. The clinical feasibility of the proposed approach was successfully demonstrated with the quantitative analysis of K(+) and Na(+) in tear samples. The concentration levels found for K(+) and Na(+) were, respectively, 20.8 ± 0.1 mM and 101.2 ± 0.1 mM for sample #1, and 20.4 ± 0.1 mM and 111.4 ± 0.1 mM for sample #2.

Simple pencil-drawn paper-based devices for one-spot electrochemical detection of electroactive species in oil samples.

We propose here simple electrochemical cells assembled with electrodes pencil drawn on paper for conducting one-spot tests enabling olive oil to be easily distinguished from other vegetable oils. They consist of small circular pads of hydrophilic paper defined by hydrophobic barriers, these last printed by using custom-designed rubber stamps, where working, reference, and counterelectrodes are drawn by pencil leads. These cells were first wetted with a small volume of aqueous electrolyte, avoiding coating of the upper surface of electrodes. A controlled volume of edible oil samples was then applied on top of the moist cell. The results found proved that these devices can be adopted as effective platforms suitable for the detection of electroactive compounds present in edible oils. In fact, they allow voltammetric profiles to be recorded not only for the oxidation of water soluble species (ortho-diphenols, as well as some monophenols and polyphenols) present in olive oils, but also for electroactive hydrophobic components (e.g., α-tocopherol) present in sunflower oils, which were chosen as model of seed oils. The whole of collected findings pointed out that simple one-spot tests performed by these devices enable olive oils to be easily distinguished from other edible oils on the basis of their clearly different voltammetric profiles. A satisfactory interdevice reproducibility (±13%) was estimated by applying strictly similar extra virgin olive oil samples onto seven different cells carefully prepared by the same procedure. An operating mechanism able to account for the detection of also electroactive hydrophobic compounds present in oils is proposed.

Pencil-drawn paper supported electrodes as simple electrochemical detectors for paper-based fluidic devices.

A simple procedure for preparing inexpensive paper-based three-electrode electrochemical cells is described here. They consist of small circular pads of hydrophilic paper defined by hydrophobic barriers printed on paper with wax-based ink. The back face of these pads is insulated by thermally laminating a polyethylene layer and working, reference and counter electrodes are drawn on paper by using commercial pencil leads. At last, a controlled volume of sample containing a supporting electrolyte was laid to soak in paper channels. Their performance was evaluated by assaying these devices as both simple cells suitable for recording voltammograms on static samples and low-cost detectors for flowing systems. Voltammetric tests, conducted by using potassium hexacyanoferrate(II) as model prototype, were also exploited for identifying the brand and softness of graphite sticks enabling paper to be marked with lines displaying the best conductivity. By taking advantage of the satisfactory information thus gained, pencil drawn electrodes were tested as amperometric detectors for the separation of ascorbic acid and sunset yellow, which were chosen as prototype electroactive analytes because they are frequently present concomitantly in several food matrices, such as soft drinks and fruit juices. This separation was performed by planar thin layer chromatography conducted on microfluidic paper-based devices prepared by patterning on filter paper two longitudinal hydrophobic barriers, once again printed with wax-based ink. Factors affecting both separation and electrochemical detection were examined and optimised, with best performance achieved by using a 20 mM acetate running buffer (pH 4.5) and by applying a detection potential of 0.9 V. Under these optimum conditions, the target analytes could be separated and detected within 6 min. The recorded peaks were well separated and characterized by good repeatability and fairly good sensitivity, thus proving that this approach is indeed suitable for rapidly assembling inexpensive and reliable electrochemical detectors for flow analysis systems.

A fully handwritten-on-paper copper nanoparticle ink-based electroanalytical sweat glucose biosensor fabricated using dual-step pencil and pen approach.

This article reports a facile fabrication of a highly sensitive enzyme-free sweat glucose sensor over Whatman filter paper substrate employing a dual-step pencil and pen approach. Initially, two pencil-drawn electrodes (PDEs) are obtained on the filter paper through the manual abrasion of an 8B graphite pencil. Then, the sensing layer is drawn over one of the PDEs using a custom-made copper (Cu) nanoparticle ink (CuNP-ink) pen to form copper nanoparticle ink decorated PDE (CuNP-ink/PDE) which serves as a working electrode, while the other PDE is drawn with silver conductive ink (Ag-ink) pen to form reference electrode. The developed CuNP-ink/PDE is composed of 43.5% of metallic Cu nanoparticles, inducing a highly crystalline and conductive nature, thus promoting fast electron transfer during glucose electrooxidation. The developed sensor offers a sensitivity of 2691.7 µAmM-1cm-2, a detection limit of 0.5 µM, a linear range of 1.2-40 µM, and a fast response time of ∼ 1.5 s. Besides, the sensor measurements are stable, reproducible, and selective in glucose sensing. Also, the reliability of the sensor is tested by comparing its performance with a UV-Visible spectrophotometer for proving its efficacy in sweat glucose detection. Experimental results evince the effectiveness of the designed sensor for glucose detection in human sweat.

Electroanalytical cells pencil drawn on PVC supports and their use for the detection in flexible microfluidic devices.

A simple, effective and low-cost technique is here presented for assembling flexible and robust electrochemical devices on transparent PVC supports, using ordinary tools, all installed on a commercial desktop digitally controlled plotter/cutter. Small diamond burs were first set up to rough precise and well defined patterns on the surface of smooth and flexible PVC transparent films. Subsequently, reference, counter and working carbon electrodes were drawn onto abraded patterns by using micropencils (4B graphite leads, 0.5 mm in diameter), in their turn installed on the plotter/cutter. The effective active working surface of electrochemical cells was then defined by a thin adhesive strip or by covering the patterned support with a suitably cut adhesive layer, depending upon whether they were intended for use in batch or drop mode. After optimization of fabrication parameters, such as pressure and speed adopted during bur abrasion and pencil drawing, the electrochemical characterization of these cells was performed by using potassium hexacyanoferrate(II) as redox probe. Voltammetric responses displayed a good inter-device reproducibility (5.6%), thus confirming the effectiveness of this easy and fast assembling strategy. These PVC-based pencil-drawn electrochemical cells were then integrated as thin-layer detectors in adhesive-tape based microfluidic channels, cut and prepared in their turn using the digitally controlled plotter/cutter. These detectors offer the advantage given by the impermeability of PVC supports, thus avoiding absorption of the flowing carrier and consequent analyte broadening, instead occurring when electrochemical cells are pencil drawn on hydrophilic materials as paper. After optimization of the complete fabrication process, the effectiveness of these devices was tested by a proof-of-concept direct quantification of ascorbic acid in commonly used drugs.

A cotton thread fluidic device with a wall-jet pencil-drawn paper based dual electrode detector.

A simple and sensitive device is presented based on the use of pencil-drawn paper based electrochemical detector placed at the end of a cotton thread fluidic channel in wall-jet configuration. This innovative and fast responding electroanalytical system can be adopted for both single and dual electrode electrochemical detection, this last achieved by applying two different potentials at two independent working electrodes drawn on the opposite faces of the paper based detector. Its performance was preliminarily optimized by adopting hexacyanoferrate(II) as probe species undergoing reversible electrochemical processes. These devices were then used for the single electrode detection of ascorbic acid in aqueous samples and the dual electrode detection of orthodiphenols in extra virgin olive oils (EVOOs). In fact, these devices enable hydrophilic orthodiphenols, typically present in EVOOs (extracted by a 80:20% v/v acetonitrile/water mixture), to be discriminated from hydrophilic monophenols instead present in almost all vegetable oils. Flow-injections runs were conducted by using a 0.01 M H2SO4 + 0.5 KCl running electrolyte allowing the rapid and selective detection of hydrophilic orthodiphenols with satisfactory sensitivity and a low enough detection limit (2 µM). Different real samples of EVOOs and sunflower oils were analyzed. Abundant enough contents of orthidiphenols were found in EVOO samples, while no trace of these antioxidants was found in sunflower oils.

A paper-based platform with a pencil-drawn dual amperometric detector for the rapid quantification of ortho-diphenols in extravirgin olive oil.

A simple, sensitive and fast responding device is described for the discrimination of hydrophilic ortho-diphenols, whose presence in abundant enough amounts is typical for extra virgin olive oils (EVOOs), from hydrophilic mono-phenols instead present in almost all vegetable oils. It consists of a dual electrode detector pencil-drawn at the end of a paper microfluidic channel, defined by hydrophobic barriers, where samples of these antioxidants, extracted from vegetable oils by a 80:20% v/v acetonitrile/water mixture, were applied. Thin-layer chromatographic runs conducted by using a 0.01 M H2SO4 + 1 M KCl running buffer allowed the selective detection of hydrophilic ortho-diphenols by profiting from the fact that they undergo reversible oxidation at less positive potentials than those required by monophenols for displaying their irreversible anodic process. On this basis, a potential for the oxidation of hydrophilic ortho-diphenols was applied to the upstream pencil-drawn electrode (W1) (at which a minor fraction of mono-phenols was also oxidized), while a potential for the reverse process involving the sole product (ortho-quinones) of the reversible oxidation of ortho-diphenols was imposed at the downstream pencil-drawn working electrode (W2). Thus, cathodic peak currents linearly dependent on analyte concentrations could be recorded at W2 which led to a satisfactory detection limit (8 µM, equivalent to 1.23 mg/L) even when working electrodes W1 and W2 with same dimensions were employed. Improved sensitivities and lower detection limits were achieved by increasing the dimensions of W2 with respect to W1, thanks to the improvement of the collection efficiency. Throughout this investigation, hydroxytyrosol (HTy) and tyrosol (Ty) were adopted as models of ortho-diphenols and mono-phenols, respectively, in view of their abundant presence in EVOOs. Real samples of EVOO from different production companies, of a simple olive oil and of a sunflower oil were analyzed. Different hydrophilic ortho-diphenol contents were found in EVOO samples (up to 40.8 mg/kg), while only a negligible amount turned out to be present in simple olive oil. No trace of these antioxidants were instead found in sunflower oil, as expected. All concentrations found were in good agreement with those detected by a more frequently employed spectrophotometric method used for the sake of comparison.

Pencil It in: Exploring the Feasibility of Hand-Drawn Pencil Electrochemical Sensors and Their Direct Comparison to Screen-Printed Electrodes.

We explore the fabrication, physicochemical characterisation (SEM, Raman, EDX and XPS) and electrochemical application of hand-drawn pencil electrodes (PDEs) upon an ultra-flexible polyester substrate; investigating the number of draws (used for their fabrication), the pencil grade utilised (HB to 9B) and the electrochemical properties of an array of batches (i.e, pencil boxes). Electrochemical characterisation of the PDEs, using different batches of HB grade pencils, is undertaken using several inner- and outer-sphere redox probes and is critically compared to screen-printed electrodes (SPEs). Proof-of-concept is demonstrated for the electrochemical sensing of dopamine and acetaminophen using PDEs, which are found to exhibit competitive limits of detection (3σ) upon comparison to SPEs. Nonetheless, it is important to note that a clear lack of reproducibility was demonstrated when utilising these PDEs fabricated using the HB pencils from different batches. We also explore the suitability and feasibility of a pencil-drawn reference electrode compared to screen-printed alternatives, to see if one can draw the entire sensing platform. This article reports a critical assessment of these PDEs against that of its screen-printed competitors, questioning the overall feasibility of PDEs' implementation as a sensing platform.

A simple method for patterning poly(dimethylsiloxane) barriers in paper using contact-printing with low-cost rubber stamps.

This paper presents a simple and low-cost method for patterning poly(dimethylsiloxane) (PDMS) barriers in porous support such as paper for the construction of flexible microfluidic paper-based analytical devices (µPADs). The fabrication method consisted of contact-printing a solution of PDMS and hexane (10:1.5 w/w) onto chromatographic paper using custom-designed rubber stamps containing the patterns of µPADs. After penetrating the paper (∼30 s), the PDMS is cured to form hydrophobic barriers. Under optimized conditions, hydrophobic barriers and hydrophilic channels with dimensions down to 949±88 µm and 771±90 µm (n=5), respectively, were obtained. This resolution is well-suitable for most applications in analytical chemistry. Chemical compatibility studies revealed that the PDMS barriers were able to contain some organic solvents, including acetonitrile and methanol, and aqueous solutions of some surfactants. This find is particularly interesting given that acetonitrile and methanol are the most used solvents in chromatographic separations, non-aqueous capillary electrophoresis and electroanalysis, as well as aqueous solutions of surfactants are suitable mediums for cell lyses assays. The utility of the technique was evaluated in the fabrication of paper-based electrochemical devices (PEDs) with pencil-drawn electrodes for experiments in static cyclic voltammetry and flow injection analysis (FIA) with amperometric detection, in both aqueous and organic mediums.