3D-printed electrodes using graphite/carbon nitride/polylactic acid composite material: A greener platform for detection of amaranth dye in food samples.

The production of sustainable materials with properties aimed at the additive manufacturing of electrochemical sensors has gained prestige in the scientific scenario. Here, a novel lab-made composite material using graphite (G) and carbon nitride (C3N4) embedded into polylactic acid (PLA) biopolymer is proposed to produce 3D-printed electrodes. PLA offers printability and mechanical stability in this composition, while G and C3N4 provide electrical properties and electrocatalytic sites, respectively. Characterizations by Raman and infrared spectroscopies and Energy Dispersive X-rays indicated that the G/C3N4/PLA composite was successfully obtained, while electron microscopy images revealed non-homogeneous rough surfaces. Better electrochemical properties were achieved when the G/C3N4/PLA proportion (35:5:60) was used. As a proof of concept, amaranth (AMR), a synthetic dye, was selected as an analyte, and a fast method using square wave voltammetry was developed. Utilizing the 3D-printed G/C3N4/PLA electrode, a more comprehensive linear range (0.2 to 4.2 µmol/L), a 5-fold increase in sensitivity (9.83 µmol-1 L µA), and better limits of detection (LOD = 0.06 µmol/L) and quantification (LOQ = 0.18 µmol/L) were achieved compared to the G/PLA electrode. Samples of jelly, popsicles, isotonic drinks, and food flavoring samples were analyzed, and similar results to those obtained by UV-vis spectrometry confirmed the method's reliability. Therefore, the described sensor is a simple, cost-effective alternative for assessing AMR in routine food analysis.

CO2-plasma surface treatment of graphite sheet electrodes for detection of chloramphenicol, ciprofloxacin and sulphanilamide.

Graphite sheet (GS) electrodes are flexible and versatile substrates for sensing electrochemical; however, their use has been limited to incorporate (bio)chemical modifiers. Herein, we demonstrated that a cold (low temperature) CO2 plasma treatment of GS electrodes provides a substantial improvement of the electrochemical activity of these electrodes due to the increased structural defects on the GS surface as revealed by Raman spectroscopy (ID/IG ratio), and scanning electron microscopy images. XPS analyses confirmed the formation of oxygenated functional groups at the GS surface after the plasma treatment that are intrinsically related to the substantial increase in the electron transfer coefficient (K0 values increased from 1.46 × 10-6 to 2.09 × 10-3 cm s-1) and with reduction of the resistance to charge transfer (from 129.8 to 0.251 kΩ). The improved electrochemical activity of CO2-GS electrodes was checked for the detection of emerging contaminant species, such as chloramphenicol (CHL), ciprofloxacin (CIP) and sulphanilamide (SUL) antibiotics, at around + 0.15, + 1.10 and + 0.85 V (versus Ag/AgCl), respectively, by square wave voltammetry. Limit of detection values in the submicromolar range were achieved for CHL (0.08 µmol L-1), CIP (0.01 µmol L-1) and SFL (0.11 µmol L-1), which enabled the sensor to be successfully applied to natural waters and urine samples (recovery values from 85 to 119%). The CO2-GS electrode is highly stable and inexpensive ($0.09 each sensor) and can be easily inserted in portable 3D printed cells for environmental on-site analyses.

Nickel Oxy-Hydroxy/Multi-Wall Carbon Nanotubes Film Coupled with a 3D-Printed Device as a Nonenzymatic Glucose Sensor.

A rapid and simple method for the amperometric determination of glucose using a nanocomposite film of nickel oxyhydroxide and multi-walled carbon nanotube (MWCNTs) was evaluated. The NiHCF)/MWCNT electrode film was fabricated using the liquid-liquid interface method, and it was used as a precursor for the electrochemical synthesis of nickel oxy-hydroxy (Ni(OH)2/NiOOH/MWCNT). The interaction between nickel oxy-hydroxy and the MWCNTs provided a film that is stable over the electrode surface, with high surface area and excellent conductivity. The nanocomposite presented an excellent electrocatalytic activity for the oxidation of glucose in an alkaline medium. The sensitivity of the sensor was found to be 0.0561 µA µmol L-1, and a linear range from 0.1 to 150 µmol L-1 was obtained, with a good limit of detection (0.030 µmol L-1). The electrode exhibits a fast response (150 injections h-1) and a sensitive catalytic performance, which may be due to the high conductivity of MWCNT and the increased active surface area of the electrode. Additionally, a minimal difference in the slopes for ascending (0.0561 µA µmol L-1) and descending (0.0531 µA µmol L-1) was observed. Moreover, the sensor was applied to the detection of glucose in artificial plasma blood samples, achieving values of 89 to 98% of recovery.

A simple, fast, portable and selective system using carbon nanotubes films and a 3D-printed device for monitoring hydroxychloroquine in environmental samples.

In this work, an electrochemical method was developed for rapid and sensitive detection of hydroxychloroquine (HCQ), an ineffective candidate drug for COVID-19 treatment however widely consumed during the pandemic, in aqueous samples using a multi-walled carbon nanotubes (MWCNT) film produced through the interfacial method on the indium tin oxide electrode (ITO). According to Raman spectroscopy, X-ray diffraction, UV-vis spectroscopy, Energy-dispersive X-ray spectroscopy, scanning electron microscopy, and atomic force microscopy, the interfacial method produces homogeneous thin films of carbon nanotubes on the substrate surface, which keep connected to the surface forming a three-dimensional microporous structure. The electrochemical behavior and oxidation kinetics of HCQ were also investigated in the MWCNT film. The sensor showed a 7 times higher oxidation current for (69.88 µA) for HCQ than the ITO electrode (9.33 µA) due to the electrocatalytic properties MWCNTs. The ITO-modified electrode was assembled on a portable 3D-printed batch-injection cell for the amperometric detection of HCQ. The oxidation peak current of HCQ is linearly proportional to the concentrations of HCQ ranging from 1.0 to 100.0 µmol L-1, with a limit of detection of 0.27 µmol L-1. Water samples (river and tap water) were spiked with HCQ, without the need for dispendious pretreatment (except filtration), and analyzed by the portable system, revealing the detection of HCQ with the recovery of 92.0%-99.8%, which suggested the great potential for real environmental monitoring application.

Prussian blue-modified laser-induced graphene platforms for detection of hydrogen peroxide.

A laser-induced graphene (LIG) surface modified with Prussian blue (iron hexacyanoferrate) is demonstrated as a novel electrochemical sensing platform for the sensitive and selective detection of hydrogen peroxide. Electrochemical Prussian blue (PB) modification on porous graphene films engraved by infrared laser over flexible polyimide was accomplished. Scanning electron microscopy images combined with Raman spectra confirm the formation of porous graphene and homogenous electrodeposition of PB over this porous surface. Electrochemical impedance spectroscopy reveals a substantial decrease in the resistance to charge transfer values (from 395 to 31.4 Ω) after the PB insertion, which confirms the formation of a highly conductive PB-graphene composite. The synergistic properties of PB and porous graphene were investigated for the constant monitoring of hydrogen peroxide at 0.0 V vs. Ag|AgCl|KCl(sat.), under high-flow injections (166 µL s-1) confirming the high stability of the modified surface and fast response within a wide linear range (from 1 to 200 µmol L-1). Satisfactory detection limit (0.26 µmol L-1) and selectivity verified by the analysis of complex samples confirmed the excellent sensing performance of this platform. We highlight that the outstanding sensing characteristics of the developed sensor were superior in comparison with other PB-based or LIG-based electrochemical sensors reported for hydrogen peroxide detection.

Highly-sensitive voltammetric detection of trinitrotoluene on reduced graphene oxide/carbon nanotube nanocomposite sensor.

This work presents the highly-sensitive detection of 2,4,6-trinitrotoluene (TNT) on reduced graphene oxide/multi-walled carbon nanotube (rGO/MWCNT) nanocomposite sensor. The formation of a thin film of this nanocomposite occurred at the cyclohexane/water immiscible interface of a mixture of MWCNT and rGO in the biphasic solution. The film was transferred to a boron-doped diamond (BDD) electrode for the square-wave voltammetric detection of TNT, which presented improved analytical characteristics in comparison with bare BDD and after modification with precursors. Electrochemical impedance spectroscopy also revealed the faster electron transfer for a redox probe on the nanocomposite modified surface. The synergistic properties of both carbon nanomaterials in the thin film modified surface resulted in a TNT sensor with a detection limit of 0.019 µmol L-1 within a wide linear range (0.5-1100 µmol L-1), with superior performance in comparison with other electrochemical sensors produced with carbon nanomaterials. This new material provides great promises for the highly-sensitive detection of other nitroaromatic explosives as well as other analytes. Moreover, the interfacial method enables the production of homogeneous and stable films on large coated areas as well as the large-scale production of electrochemical sensors.

3D printing for electroanalysis: From multiuse electrochemical cells to sensors.

This work presents potential applications of low-cost fused deposition modeling 3D-printers to fabricate multiuse 3D-printed electrochemical cells for flow or batch measurements as well as the 3D-printing of electrochemical sensing platforms. Electrochemical cells and sensors were printed with acrylonitrile butadiene styrene (ABS) and conductive graphene-doped polylactic acid (G-PLA) filaments, respectively. The overall printing operation time and estimated cost per cell were 6 h and $ 6.00, respectively, while the sensors were printed within minutes (16 sensor strips of 1 × 2 cm in 10 min at a cost of $ 1.00 each sensor). The cell performance is demonstrated for the amperometric detection of tert-butylhydroquinone, dipyrone, dopamine and diclofenac by flow-injection analysis (FIA) and batch-injection analysis (BIA) using different working electrodes, including the proposed 3D-printed sensor, which presented comparable electroanalytical performance with other carbon-based electrodes (LOD of 0.1 µmol L-1 for dopamine). Raman spectroscopy and scanning electron microscopy of the 3D-printed sensor indicated the presence of graphene nanoribbons within the polymeric matrix. Electrochemical impedance spectroscopy and heterogeneous electron transfer constants (k0) for the redox probe Ru(NH3)6+3 revealed that a glassy-carbon electrode presented faster electron transfer rates than the 3D-printed sensor; however, the latter presented lower LOD values for dopamine and catechol probably due to oxygenated functional groups at the G-PLA surface.

Carbon nanotube/Prussian blue thin films as cathodes for flexible, transparent and ITO-free potassium secondary battery.

Thin films of either unpurified single-walled carbon nanotubes (SWCNT) or iron-filled multi-walled carbon nanotubes (MWCNT) were deposited through the liquid-liquid interfacial route over plastic substrates, yielding transparent, flexible and ITO-free electrodes. The iron species presented in both electrodes (inside of the MWCNT cavities or outside of the SWCNT bundles, related to the catalyst remaining of the growth process) were employed as reactant to the electrosynthesis of Prussian blue (PB), yielding carbon nanotubes/Prussian blue nanocomposite thin films, which were characterized by Raman spectroscopy, scanning electron microscopy, atomic force microscopy, cyclic voltammetry and galvanostatic charge/discharge measurements. The nanocomposite films were employed as cathodes for flexible, transparent and ITO-free potassium batteries, showing reversible charge/discharge behavior and specific capacitance of 8.3mAhcm(-3) and 2.7mAhcm(-3) for SWCNT/PB and MWCNT/PB, respectively.