Selective Detection of Paraquat in Adulterated and Complex Environmental Samples Using Raman Spectroelectrochemistry.

Growing demand for pesticides has created an environment prone to deceptive activities, where counterfeit or adulterated pesticide products infiltrate the market, often escaping rapid detection. This situation presents a significant challenge for sensor technology, crucial in identifying authentic pesticides and ensuring agricultural safety practices. Raman spectroscopy emerges as a powerful technique for detecting adulterants. Coupling the electrochemical techniques allows a more specific and selective detection and compound identification. In this study, we evaluate the efficacy of spectroelectrochemical measurements by coupling a potentiostat and Raman spectrograph to identify paraquat, a nonselective herbicide banned in several countries. Our findings demonstrate that applying -0.70 V during measurements yields highly selective Raman spectra, highlighting the primary vibrational bands of paraquat. Moreover, the selective Raman signal of paraquat was discernible in complex samples, including tap water, apple, and green cabbage, even in the presence of other pesticides such as diquat, acephate, and glyphosate. These results underscore the potential of this technique for reliable pesticide detection in diverse and complex matrices.

Investigating layer-by-layer films of carbon nanotubes and nickel phthalocyanine towards diquat detection.

The indiscriminate use of pesticides makes us susceptible to the toxicity of these chemical compounds, which may be present in high quantities in our food. It is crucial to develop inexpensive and rapid methods for determining these pesticides for government control or even for the general population. In this study, we investigated the fabrication of self-assembled LbL films using multi-walled carbon nanotubes (MWCNT) and nickel tetrasulphonated phthalocyanine (NiTsPc) as an electrochemical sensor for the herbicide Diquat (DQ). The Layer-by-Layer (LbL) assembly of the (MWCNT/NiTsPc) film was examined, along with its structural and morphological characteristics. The effect of the number of layers in DQ detection was evaluated by cyclic voltammetry, followed by the detection through differential pulse voltammetry. The achieved limit of detection was 9.62 × 10-7 mol L-1. A ~ 30% decrease in sensitivity was observed in the presence of Paraquat, a banned herbicide and electrochemical interferent due to the structural similarities, which is regularly neglected in the most published studies. The sensor was tested in real samples, demonstrating a recovery of 98.5% in organic apples.

Titanium-Based Alloy Surface Modification with TiO2 and Poly(sodium 4-styrenesulfonate) Multilayers for Dental Implants.

Implant placement is an important repair method in dentistry and orthopedics. Increasing efforts have focused on optimizing the biocompatibility and osseointegration properties of titanium (Ti) and Ti-based alloys. In this work, Ti-based alloys were modified by the layer-by-layer (LbL) technique, which is a simple and versatile method for surface modification. The morphology and chemical structure of LbL films of poly(sodium 4-styrenesulfonate) (PSS) and Ti dioxide (TiO2) nanoparticles were first characterized employing ultraviolet-visible and Fourier-transform infrared spectroscopies as well as atomic force microscopy for further application in Ti-based alloy implants. The changes provoked by the LbL PSS/TiO2 film on the Ti-based alloy surfaces were then investigated by scanning electron microscopy and micro-Raman techniques. Finally, in vivo tests (immunolabeling and biomechanical analysis) performed with screw implants in rats suggested that PSS/TiO2 multilayers promote changes in both topography and chemical surface properties of the screw, providing beneficial effects for osteoblast activity. This simple and relatively low-cost growth process can open up possibilities to improve dental implants and, probably, bone implants in general.

Improving direct immunoassay response by layer-by-layer films of gold nanoparticles - Antibody conjugate towards label-free detection.

The conjugation of nanoparticles with antibodies has been successfully applied in sandwich immunoassays for detecting cancer biomarkers. However, two antibodies are necessary to perform such experiment, being one of them functionalized with a signal label for optical or electrochemical assay. This approach is time and cost consuming compared to direct label-free immunoassays. In this study, we propose the synthesis of gold nanoparticles conjugated to anti-PSA antibody to produce a label-free impedimetric immunosensors based on nanostructured Layer-by-Layer (LbL) films. Prostate-specific antigen (PSA) detection was performed by electrochemical impedance spectroscopy demonstrating a detection mechanism governed by Langmuir-Freundlich adsorption model. This strategy provided a significant sensitivity using 10-fold less antibody than conventional immunosensors, i.e. decreasing costs using a simple approach, with a limit of detection of 0.17 ng mL-1 and an analytical range of 0.1-20 ng mL-1 indicating that our sensor is potentially useful for clinical applications.

Layer-by-Layer Films of Gold Nanoparticles and Carbon Nanotubes for Improved Amperometric Detection of Cholesterol.

Distinct architectures of layer-by-layer (LbL) films made of carbon nanotubes and gold nanoparticles were investigated to serve as the matrix to immobilize cholesterol oxidase, with which cholesterol could be detected using amperometry. The gold nanoparticles were synthesized by metal reduction stabilized in poly(allylamine hydrochloride) (PAH) providing a stable AuNPs-PAH suspension, while multi-walled carbon nanotubes (CNTs) were functionalized with carboxylic groups to obtain an aqueous suspension. The LbL films were deposited on ITO, with a cushion film of PAH and poly(vinyl sulfonic acid) (PVS). Owing to the synergy between CNTs and AuNPs, the electrode ITO/(PAH/PVS)2 (AuNPs-PAH/CNTs)10 was selected for immobilization of cholesterol oxidase (ChOx). This sensor could detect cholesterol with a limit of detection of 14.8 µmol L-1 and sensitivity of 36.47 (µA cm-2)·(mmol L-1)-1. It was also able to determine cholesterol in egg yolk with a recovery of 97.7%.

Bending of layer-by-layer films driven by an external magnetic field.

We report on optimized architectures containing layer-by-layer (LbL) films of natural rubber latex (NRL), carboxymethyl-chitosan (CMC) and magnetite (Fe3O4) nanoparticles (MNPs) deposited on flexible substrates, which could be easily bent by an external magnetic field. The mechanical response depended on the number of deposited layers and was explained semi-quantitatively with a fully atomistic model, where the LbL film was represented as superposing layers of hexagonal graphene-like atomic arrangements deposited on a stiffer substrate. The bending with no direct current or voltage being applied to a supramolecular structure containing biocompatible and antimicrobial materials represents a proof-of-principle experiment that is promising for tissue engineering applications in biomedicine.

Recent advances in electronic tongues.

This minireview describes the main developments of electronic tongues (e-tongues) and taste sensors in recent years, with a summary of the principles of detection and materials used in the sensing units. E-tongues are sensor arrays capable of distinguishing very similar liquids employing the concept of global selectivity, where the difference in the electrical response of different materials serves as a fingerprint for the analysed sample. They have been widely used for the analysis of wines, fruit juices, coffee, milk and beverages, in addition to the detection of trace amounts of impurities or pollutants in waters. Among the various principles of detection, electrochemical measurements and impedance spectroscopy are the most prominent. With regard to the materials for the sensing units, in most cases use is made of ultrathin films produced in a layer-by-layer fashion to yield higher sensitivity with the advantage of control of the film molecular architecture. The concept of e-tongues has been extended to biosensing by using sensing units capable of molecular recognition, as in films with immobilized antigens or enzymes with specific recognition for clinical diagnosis. Because the identification of samples is basically a classification task, there has been a trend to use artificial intelligence and information visualization methods to enhance the performance of e-tongues.