E-Tongues/Noses Based on Conducting Polymers and Composite Materials: Expanding the Possibilities in Complex Analytical Sensing

Electronic Tongues and Noses: A General Overview

As technology advances, electronic tongues and noses are becoming increasingly important in various industries. These devices can accurately detect and identify different substances and gases based on their chemical composition. This can be incredibly useful in fields such as environmental monitoring and industrial food applications, where the quality and safety of products or ecosystems should be ensured through a precise analysis. Traditionally, this task is performed by an expert panel or by using laboratory tests but sometimes becomes a bottleneck because of time and other human factors that can be solved with technologies such as the provided by electronic tongue and nose devices. Additionally, these devices can be used in medical diagnosis, quality monitoring, and even in the automotive industry to detect gas leaks. The possibilities are endless, and as these technologies continue to improve, they will undoubtedly play an increasingly important role in improving our lives and ensuring our safety. Because of the multiple applications and developments in this field in the last years, this work will present an overview of the electronic tongues and noses from the point of view of the approaches developed and the methodologies used in the data analysis and steps to this aim. In the same manner, this work shows some of the applications that can be found in the use of these devices and ends with some conclusions about the current state of these technologies.

Recent Advances and Future Perspectives in the E-Nose Technologies Addressed to the Wine Industry

Electronic nose devices stand out as pioneering innovations in contemporary technological research, addressing the arduous challenge of replicating the complex sense of smell found in humans. Currently, sensor instruments find application in a variety of fields, including environmental, (bio)medical, food, pharmaceutical, and materials production. Particularly the latter, has seen a significant increase in the adoption of technological tools to assess food quality, gradually supplanting human panelists and thus reshaping the entire quality control paradigm in the sector. This process is happening even more rapidly in the world of wine, where olfactory sensory analysis has always played a central role in attributing certain qualities to a wine. In this review, conducted using sources such as PubMed, Science Direct, and Web of Science, we examined papers published between January 2015 and January 2024. The aim was to explore prevailing trends in the use of human panels and sensory tools (such as the E-nose) in the wine industry. The focus was on the evaluation of wine quality attributes by paying specific attention to geographical origin, sensory defects, and monitoring of production trends. Analyzed results show that the application of E-nose-type sensors performs satisfactorily in that trajectory. Nevertheless, the integration of this type of analysis with more classical methods, such as the trained sensory panel test and with the application of destructive instrument volatile compound (VOC) detection (e.g., gas chromatography), still seems necessary to better explore and investigate the aromatic characteristics of wines.

Modification of bismuth film and poly(L-hydroxyproline) onto a screen-printed graphene electrode: A step towards a non-enzymatic platform for determining L-hydroxyproline in human urine samples

A novel sensing material for L-hydroxyproline (Hyp) detection was developed by synthesizing a bismuth film (BiF) and poly(L-hydroxyproline) (Poly(Hyp)) on a screen-printed graphene electrode (SPGE). Initially, the BiF electrodeposition was produced on the SPGE surface, followed by the Poly(Hyp) electropolymerization, resulting in the obtained Poly(Hyp)/BiF/SPGE. The morphology of a sponge-like thin film of Poly(Hyp)/BiF on SPGE had uniform nanometer-sized cavities on the graphene surface, offering a large electroactive area for interaction with the target substance. Cyclic and differential pulse voltammetry was used to examine the sensing performance of the proposed sensor, which revealed that the Poly(Hyp)/BiF/SPGE had the highest response toward Hyp detection. This was attributed to BiF and Poly(Hyp) can facilitate the transfer of electrons at an electrode/solution interface, resulting in an effective sensor for the detection of Hyp. Under the optimal conditions, the quantification of the proposed sensor was found to be linearly related to Hyp concentrations in the range of 0.01-5.0 mM with a limit of detection of 9.2 μM. Moreover, the interference of other substances detected in biological fluids showed no effect based on the ±5% error, indicating good selectivity for Hyp detection. In real applications, the proposed assay successfully analyzed Hyp in human urine samples, yielding satisfactory results with recoveries in the acceptable range of 98%-102%. Therefore, this facilely synthesized approach could be a suitable candidate to obtain a material to fabricate a new sensor for the measurement of Hyp, an important biomarker in the human body.

Self-Healing E-tongue

Self-healing materials inspire the next generation of multifunctional wearables and Internet of Things appliances. They expand the realm of thin film fabrication, enabling seamless conformational coverage irrespective of the shape complexity and surface geometry for electronic skins, smart textiles, soft robotics, and energy storage devices. Within this context, the layer-by-layer (LbL) technique is versatile for homogeneously dispersing materials onto various matrices. Moreover, it provides molecular level thickness control and coverage on practically any surface, with poly(ethylenimine) (PEI) and poly(acrylic acid) (PAA) being the most used materials primarily employed in self-healing LbL structures operating at room temperature. However, achieving thin film composites displaying controlled conductivity and healing ability is still challenging under ambient conditions. Here, PEI and PAA are mixed with conductive fillers (gold nanorods, poly(3,4-ethylene dioxythiophene): polystyrenesulfonate (PEDOT:PSS), reduced graphene oxides, and multiwalled carbon nanotubes) in distinct LbL film architectures. Electrical (AC and DC), optical (Raman spectroscopy), and mechanical (nanoindentation) measurements are used for characterizing composite structures and properties. A delicate balance among electrical, mechanical, and structural characteristics must be accomplished for a controlled design of conductive self-healing composites. As a proof-of-concept, four LbL composites were chosen as sensing units in the first reported self-healing e-tongue. The sensor can easily distinguish basic tastes at low molar concentrations and differentiate trace levels of glucose in artificial sweat. The formed nanostructures enable smart coverages that have unique features for solving current technological challenges.

Bacteria-Infected Artificial Urine Characterization Based on a Combined Approach Using an Electronic Tongue Complemented with 1H-NMR and Flow Cytometry

The prevailing form of bacterial infection is within the urinary tract, encompassing a wide array of bacteria that harness the urinary metabolome for their growth. Through their metabolic actions, the chemical composition of the growth medium undergoes modifications as the bacteria metabolize urine compounds, leading to the subsequent release of metabolites. These changes can indirectly indicate the existence and proliferation of bacterial organisms. Here, we investigate the use of an electronic tongue, a powerful analytical instrument based on a combination of non-selective chemical sensors with a partial specificity for data gathering combined with principal component analysis, to distinguish between infected and non-infected artificial urine samples. Three prevalent bacteria found in urinary tract infections were investigated, Escherichia coli, Klebsiella pneumoniae, and Enterococcus faecalis. Furthermore, the electronic tongue analysis was supplemented with 1H NMR spectroscopy and flow cytometry. Bacteria-specific changes in compound consumption allowed for a qualitative differentiation between artificial urine medium and bacterial growth.

Quick classification of strong-aroma types of base Baijiu using potentiometric and voltammetric electronic tongue combined with chemometric techniques

Nowadays, the classification of strong-aroma types of base Baijiu (base SAB) is mainly achieved by human sensory evaluation. However, prolonged tasting brings difficulties for sommeliers in guaranteeing the consistency of results, and may even cause health problems. Herein, an electronic tongue (E-Tongue) combined with a gas chromatography-mass spectrometry (GC-MS) method was successfully developed to grade high-alcoholic base SAB. The E-tongue was capable of identifying base SAB samples into four grades by a discriminant function analysis (DFA) model based on human sensory evaluation results. More importantly, it could effectively and rapidly predict the quality grade of unknown base SAB with an average accuracy up to 95%. The differences of chemical components between base SAB samples were studied by the GC-MS analysis and 52 aroma compounds were identified. The qualitative and quantitative results showed that with the increase of base SAB grade, the varieties and contents of aroma compounds increased. Overall, the comprehensive analysis of E-tongue data and GC-MS results could be in good agreement with human sensory evaluation results, which also proved that the newly developed method has a potential to be a useful alternative to the overall quality grading of base Baijiu.

Electronic Nose and Tongue for Assessing Human Microbiota

The technological developments of recent times have allowed the use of innovative approaches to support the diagnosis of various diseases. Many of such clinical conditions are often associated with metabolic unbalance, in turn producing an alteration of the gut microbiota even during asymptomatic stages. As such, studies regarding the microbiota composition in biological fluids obtained by humans are continuously growing, and the methodologies for their investigation are rapidly changing, making it less invasive and more affordable. To this extent, Electronic Nose and Electronic Tongue tools are gaining importance in the relevant field, making them a useful alternative—or support—to traditional analytical methods. In light of this, the present manuscript seeks to investigate the development and use of such tools in the gut microbiota assessment according to the current literature. Significant gaps are still present, particularly concerning the Electronic Tongue systems, however the current evidence highlights the strong potential such tools own to enter the daily clinical practice, with significant advancement concerning the patients’ acceptability and cost saving for healthcare providers.

Polyaniline Nanofibers-Embedded Gold Nanoparticles Obtained by Template-Free Procedure with Immobilization Prospects

In this work, template-free nanostructured conducting polymers (nCPs)-embedded gold nanoparticles (AuNPs) from aniline, thiophene and 3,4-ethylenedioxythiophene have been prepared via a one-pot sonochemical method. The synthesis of the nanocomposite (nCPs-AuNPs) was achieved in a short period of time (5-10 min), by applying high-energy ultrasound to an aqueous mixture of a CP precursor monomer and KAuCl4, in the presence of LiClO4 as dopant. The synthesis process is simpler, greener and faster in comparison to other procedures reported in the literature. Remarkably, bulk quantities of doped polyaniline PANI-AuNPs nanofibers were obtained. Subsequently, they were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR), as well as by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). PANI-AuNPs nanofibers were also employed as immobilization matrix for a benchmark enzyme, glucose oxidase (GOX). Finally, glucose was determined in real samples of white and red wines by using the so-obtained GOX-PANI-AuNPs/Sonogel-Carbon biosensor, providing outstanding recoveries (99.54%). This work may offer important insights into the synthesis of nanostructured conducting polymers and also stimulates the exploration of the applications of these nanocomposites, especially in research fields such as (bio)sensors, catalysis and composite materials.