A team of researchers recently published a paper in the journal Analytica Chimica Acta that reviewed the use of conjugated polymer-based electronic tongues (CPETs) in different applications in the food and beverage industry.
Study: A review on conjugated polymer-based electronic tongues. Image Credit: Zametalov/Shutterstock.com
Electronic tongues are analytical technologies primarily developed to mimic biological tongues that have cross-sensitive, low-selective, and non-specific taste systems. An array of sensors in the electronic tongues produce electrical signals that correspond to specific chemical compositions of a sample solution.
A series of chemoreceptors in electronic tongues generate recognition signals on detecting a chemical composition, which are then converted into electrical data and transmitted to the data processing part. In the data processing part, the information is analyzed using chemometric methods, such as principal component analysis (PCA), and converted into comprehensible quantitative or qualitative information.
The efficiency and performance of electronic tongues have been optimized for several years through the development of novel technologies and materials. Different conjugated polymers (CPs), specifically poly(3,4-ethylenedioxythiophene) (PEDOT), polythiophene (PT), polyaniline (PANI), and polypyrrole (PPy), have received significant attention for application in electronic tongues owing to their good environmental stability, cost-effective and simple preparation, and controllable electrical properties, which considerably enhanced their versatility compared to other types of electronic tongues.
In this study, researchers reviewed electronic tongues based on PEDOT, PT, PANI, and PPy that were investigated in studies in the last two decades.
PPy, a conducting polymer, is obtained by the oxidative polymerization of pyrrole, a five-membered heterocyclic organic compound. Among the CPs, PPy has been investigated extensively owing to its high conductivity and low oxidation potential. PPy is also suitable for biomedical applications, such as immobilizing substrates for biosensors and biomolecules, due to its biodegradation, thermal stability, and biocompatibility.
Several PPy-based sensors were developed for electronic tongues using three approaches to enhance the electronic tongues' stability, cross-selectivity, and characteristic electrochemical responses. These approaches include using electroactive doping agents such as hexacyanoferrate and phosphotungstate, using doubly charged anions such as sulfate, and large doping anions such as p-toluenesulfonic acid and sodium 1-decanesulfonate.
In a study, a hybrid electronic tongue synthesized using voltammetric electrodes were utilized to differentiate red wines from one another. Six different doping anions were used to modify the PPy-based sensors used in the tongue. The tongue displayed a variety of reproducible and stable responses and a robust ability to distinguish various red wines depending on their vintage, grape variety, and origin.
In another study, the aging of red wines kept in oak barrels was monitored using the PPy electronic tongue after three and six months. The electronic tongue successfully discriminated variations in the red wines caused by the oak barrels of various toasting levels. The pH of the red wines and the presence of antioxidants, electroactive molecules, and ions in them were detected efficiently using the voltammetric electronic tongue.
An array of sensors synthesized using various doping agents and PPy was employed to predict the alcoholic strength and bitterness in beer samples. The PCA of the electrochemical signals obtained from the sensors led to the definite discrimination between non-alcoholic and alcoholic beer and between pale and dark beers.
A PPy-based electronic tongue was also used to investigate the bitterness of extra virgin olive oils (EVOOs). Every sensor generated a characteristic electrochemical signal when it was exposed to emulsions of EVOOs, which provided a high cross-selectivity level. By analyzing the obtained signals, discriminatory assessments of the six EVOOs based on their degree of bitterness were performed successfully.
A voltammetric electronic tongue based on PPy- and bisphthalocyanine-modified commercial screen-printed electrodes successfully monitored the beef product freshness. The electronic tongue also discriminated putrescine and ammonia present in the beef extract powder in trace amounts.
PANI is one of the common CPs that can be synthesized easily from aniline, a low-cost monomer. PANI displays a high theoretical conductivity, environmental stability, and extensive working potential.
Template synthesis, enzymatic synthesis, electrochemical synthesis, and chemical synthesis are commonly used to synthesize PANI. Various composite materials can be prepared using PANI with different physicochemical and mechanical properties, such as different kinds of chromism and pH sensitivity.
A PANI/ruthenium pyridine/PPy-based electronic tongue consisting of Langmuir–Blodgett (LB) films was prepared to detect sucrose, quinine, sodium chloride, and hydrochloric acid using impedance spectroscopy.
The electronic tongue effectively distinguished the variations in mineral content and salt at the µg L-1 level without requiring any difficult laboratory examination. Thus, the electronic tongue can be used to differentiate inorganic contaminants in water, red wines, and beverages.
An array of PPy and PANI-based sensors was fabricated to discriminate cava wine samples based on their aging time and dryness level. The mixed-voltammetric responses were obtained from the sensor array for different samples and then pre-processed and inputted to the linear discriminant analysis (LDA) model to classify the wine samples.
Chemiresistor-type sensors were synthesized by the potentiostatic electrodeposition of poly(3-methylthiophene), PANI, and PPy to distinguish various types of coconut oil. Repeatable and reversible sensor responses were obtained within a short response time, which were analyzed and distinguished using cluster analysis (CA) and PCA. The sensor array displayed different selectivities in the headspace of the coconut oil sample.
An electronic tongue containing disposable filter paper coated with undoped and doped PANI and PPy successfully discriminated four basic tastes, fruit juice types, and water samples, indicating the viability of using these sensing substrates for foodstuff monitoring.
PT is a polyheterocycle with considerable structural similarity to PPy in both doped and undoped forms. PT is utilized for different industrial and scientific applications owing to its excellent mechanical characteristics, good thermal resilience, ease of manufacturing, and narrower energy bandgaps.
PT can be synthesized using electrochemical and chemical polymerization. The optical and electrical properties of PTs can vary based on the doping level and doping type. The addition of functional groups or side chains to PTs can improve their stability, processability, and selectivity to a particular molecule in a given sample.
A PPy/PANI/PT-based electronic tongue was used to identify and discriminate five soybean cultivars with distinct genetic characteristics. The combination of PLS and PCA-based data analysis and electrical measurements allowed the electronic tongue to effectively distinguish five types of soybeans. Moreover, the electronic tongue response was reproducible in multiple experiments.
PEDOT is one of the most important PT derivatives and a popular CP that remains stable in the doped state. PEDOT has received significant prominence owing to its good biocompatibility, easy processing, air stability, and high electrical conductivity in the p-doped state.
A water-soluble polyelectrolyte, such as poly(styrene sulfonate) (PSS), is used as a charge-balancing dopant during the polymerization to obtain PEDOT (PSS) aqueous dispersion as PEDOT is insoluble in water. PEDOT can be polymerized through transition metal-mediated coupling of dihalo derivatives of 3,4-ethylenedioxythiophene (EDOT), electrochemical polymerization, and oxidative chemical polymerization.
An amperometric electronic tongue comprising PEDOT-modified PT was employed for the blind analysis of fruit juices. The obtained data were analyzed using PCA and the discriminating capabilities were evaluated in terms of specificities and sensitivities of their partial least squares discriminant analysis (PLS-DA) models. The modified electrode displayed an excellent discriminating capability and was identified as the only system that can differentiate juices from the same fruit of different brands.
A PEDOT(PSS) layered nanocomposite-based impedimetric electronic tongue with optimized electrocatalytic characteristics was prepared using gold nanoparticles and lutetium bisphthalocyanine nanolayers to differentiate various wines. The electronic tongue successfully distinguished five red wines with similar characteristics, such as age, vintage, and region. The sensor cross-selectivity was improved due to the higher electrocatalytic activity of the layered composite sensing materials.
To summarize, the CPETs have become effective analytical tools for classifying drinks and foodstuffs from different origins and brands, specifically in the wine industry, discriminating of trace amounts of tastants in food products, and monitoring the freshness of meat products. However, more research is required to continue the development of CPETs for food chemistry applications and introduce new CP types in the field of electronic tongues.
Vahdatiyekta, P., Zniber, M., Bobacka, J. et al. A review on conjugated polymer-based electronic tongues. Analytica Chimica Acta 2022. https://doi.org/10.1016/j.aca.2022.340114