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Paixao Group

Chemical Sensors Lab



Paper-Based Fluorescent Device - The paper that glows in the dark (Collaboration - Clemson/USA)


While thermal treatment of paper can lead to the formation of aromatic structures via hydrothermal treatment (low temperature) or pyrolysis (high temperature), neither of these approaches allow patterning the substrates. Somewhere in between these two extremes, a handful of research groups have used CO2 lasers to pattern paper and induce carbonization. However, none of the previously reported papers have focused on the possibility to form fluorescent derivatives via laser-thermal engraving. Exploring this possibility, this article describes the possibility of using a CO2 laser engraver to selectively treat paper, resulting in the formation of fluorescent compounds, similar to those present on the surface of carbon dots. To determine the most relevant variables controlling this process, 3 MM chromatography paper was treated using a standard 30 W CO2 laser engraver. Under selected experimental conditions, a blue fluorescent pattern was observed when the substrate was irradiated with UV light (365 nm). The effect of various experimental conditions (engraving speed, engraving power, and number of engraving steps) was investigated to maximize the fluorescence intensity. Through a comprehensive characterization effort, it was determined that 5-(hydroxymethyl)furfural and a handful of related compounds were formed (varying in amount) under all selected experimental conditions.














Paper-Based Electrochemical device using CO2 laser for different applications


A single‐step laser scribing process is used to pattern directly nanostructured electrodes on paper‐based devices. The facile and low‐cost technique eliminates the need for chemical reagents or controlled conditions. This process involves the use of a CO2 laser to pyrolyze the surface of the paperboard, producing a conductive porous non‐graphitizing carbon material composed of graphene sheets and composites with aluminosilicate nanoparticles. The new electrode material was extensively characterized, and it exhibits high conductivity and an enhanced active/geometric area ratio; it is thus well‐suited for electrochemical purposes. As a proof‐of‐concept, the devices were successfully employed for different analytical applications in the clinical, pharmaceutical, food, and forensic fields. The scalable and green fabrication method associated with the features of the new material is highly promising for the development of portable electrochemical devices.

SERS Paper-Based Electrochemical device using Gold Nanoparticles for Forensic Applications


Drug trafficking is a major worldwide problem. In this context, cocaine is one of the most commonly used drugs of abuse, in addition, the street cocaine sample is commonly seized adulterated with pharmaceutical compounds, the composition of the mixture provides chemical fingerprint of the samples that can assist the police intelligence department to track the distribution route of the drug; hence the development of facile cost-effective methods for determining the composition of street cocaine is an important objective. Herein we report a simple strategy for the fabrication of paper-based analytical devices (PADs) for the dual electrochemical and surface-enhanced Raman-scattering (SERS) determination of cocaine samples. Accordingly, a 2-µm-thick Au film was prepared by depositing gold nanoparticles (AuNPs) on office paper with wax-barrier templates to create nanostructured gold tracks that are mainly formed by Au (111) fcc planes as electrodes and SERS transducers. These devices were characterized by scanning electron microscopy, X-ray diffractometry, electrochemical impedance spectroscopy, and energy-dispersive X-ray spectroscopy. The optimized device is simple and inexpensive to prepare and exhibited a Raman-scattering enhancement factor of 3 x 106, a 15-fold superior electroactive area, and a 2.6-fold decrease in charge-transfer resistance when compared with a conventional Au electrode. In addition, these PADs were successfully used in a forensics scenario to screen and analyze a seized street cocaine sample, determine its chemical profile, and to identify simultaneously caffeine, paracetamol, and levamisole adulterants.

Colorimetric Paper-Based devices for Forensic Applications


In this report we introduce a novel approach for an inexpensive and disposable colorimetric paper sensor array for the detection and discrimination of five explosives – triacetone triperoxide (TATP), hexamethylene triperoxide diamine (HMTD), 4-amino-2-nitrophenol (4A2NP), nitrobenzene (NB), and picric acid (PA). The colorimetric sensor comprised a disposable paper array fabricated using a wax printer and three reagents (KI, creatinine, and aniline) that produced a unique color pattern for each explosive based on chemical interactions between the explosive species and the chemical reagents. The analytes were discriminated from one another as per the color change profiles, which were readily distinguishable after 15 min, using hierarchical clustering analysis (HCA) and principal component analysis (PCA); there were no misclassifications in any of the trials conducted. The colorimetric pattern values were extracted using a smartphone, custom-made software and a closed chamber to circumvent the illumination problems commonly found in other paper approach devices. A semi-quantitative analysis was performed and it was possible to use as low as 0.2 μg of explosives.

Colorimetric Paper-Based devices with Electrochemical pre-treatment step for Forensic Applications


We report the development of a simple, portable, low-cost, high-throughput visual colorimetric paper-based analytical device for the detection of procaine in seized cocaine samples. The interference of most common cutting agents found in cocaine samples was verified, and a novel electrochemical approach was used for sample pretreatment in order to increase the selectivity. Under the optimized experimental conditions, a linear analytical curve was obtained for procaine concentrations ranging from 5 to 60 μmol L–1, with a detection limit of 0.9 μmol L–1. The accuracy of the proposed method was evaluated using seized cocaine samples and an addition and recovery protocol.