Olive oil is a vegetable lipid highly appreciated for its beneficial effects on human health. Olive oil quality is normally assessed by chemical analysis as well as sensory analysis to detect the presence of organoleptic defects.

Two of the most important parameters that define the quality of olive oil are the free acidity, that is affected by the quality of the olives used to produce the oil as well as the production process, and the peroxide index, that is an indicator of the oil primary oxidation and is affected by the storage conditions. These chemical parameters are usually determinated by manual titration procedures that must be carried out in a laboratory by trained personnel.

In this paper, a portable sensor system to evaluate the quality grade of olive oil is presented. The system is characterized by small dimensions (11 x 15 x 5 cm), light weight (350 g), quick measurement response (30 seconds) and can be powered by USB or batteries. The working principle is based on the measurement of the electrical conductance of an emulsion between an hydroalcoholic solution (60% ethanol 40% distilled water) and the olive oil sample under test. In the case of fresh olive oil samples, characterized by low values of peroxide index, the olive oil free acidity can be estimated from the electrical conductance of the emulsion. In the case of oxidized olive oil samples, the measured electrical conductance is also function of the oxidation level due to the presence of non-volatile compounds (such as aldehydes, ketones and hydrocarbons) produced by oxidation.

Tests have been carried out on a set of 17 olive oil samples and the results have shown how the proposed system can be a low-cost alternative to standard laboratory analysis to evaluate the quality grade of olive oil.

Hesperidin is the major flavonoid of Citrus L. fruits with a wide spectrum of biological activity that is caused its application in medicine. Thus, simple, sensitive and selective methods for hesperidin determination are required. The electrochemical sensors can be successfully used for these purposes due to ability of hesperidin to be oxidized on the electrode surface. Electrochemically inert tin(IV) oxide nanoparticles in combination with surfactants has been used as sensitive layer of the sensor for hesperidin. The effect of surfactants nature and concentration on hesperidin response has been evaluated. The oxidation potentials are cathodically shifted. Hesperidin oxidation currents are statistically significantly increased in the surfactants concentration range of 10-500 µM. The higher oxidation currents are obtained in the case of cationic surfactants. The best hesperidin response has been registered on the sensor based on the tin(IV) oxide dispersed in 500 µM cetylpyridinium bromide. The SEM and electrochemical methods data confirm the increase of sensor surface effective area and electron transfer rate. Hesperidin quantification using sensor developed has been performed in adsorptive differential pulse mode. Sensor gives linear response to hesperidin in the range of 0.10-10 and 10-75 µM with the detection limit of 77 nM. The analytical characteristics obtained are comparable to other sensors based on the modified electrodes but the sensor developed is simpler and less tedious in preparation. Another important advantage is high selectivity to hesperidin in presence of other flavonoids and phenolic acids. The sensor has been successfully tested on orange juices and validated with ultra-HPLC. Novel voltammetric sensor is highly sensitive and selective, reliable and can be recommended for the preliminary screening of citrus juices.

The relative humidity (RH) sensing response of a chemoresistive sensor using a novel ternary hybrid nanocomposite film as sensing element is presented. The sensitive layer was obtained using drop-casting technique for depositing a thin film of nanocomposite between the electrodes of an interdigitated structure. The sensing support structure consists of interdigitated (IDT) dual-comb structure made of Si and covered by a SiO₂ layer of 1 µm thickness. The interdigitated electrode structure is made of a chromium adhesion layer (10 nm thickness) and a gold layer (100 nm thickness).

The sensing capability of the novel thin film based on a ternary hybrid made of oxidized carbon nanohorns – titanium dioxide – polyvinylpyrrolidone (CNHox/TiO₂/PVP) nanocomposite was investigated by applying a direct current with known intensity between the two electrodes of the sensing structure, and measuring the resulting voltage difference, while varying the RH from 0% to 100% in humid nitrogen atmosphere. The ternary hybrid-based thin film's resistance increases when the sensors were exposed to relative humidity ranging from 0–100%. It was found that performances of the new chemoresistive sensor are consistent with those of the capacitive commercial sensor used as benchmark throughout the moisture monitoring experiment.

Raman spectroscopy has been used to provide information about the composition of the sensing layer, as well as about potential interactions between constituents. Several sensing mechanisms were considered and discussed, based on the interaction of water molecules with each component of the ternary nanohybrid. The sensing results obtained lead to the conclusion that the synergic effect of p-type semiconductor behavior of CNHox and the PVP swelling process plays a pivotal role for the overall resistance decrease of the sensitive film.

Extensive use of rare earth metals (REMs) in different industries has increased the concerns on the environmental fate of these elements. Accumulation of these metals in the food chain is detrimental to plants, animals, and human health. Therefore, the determination of REMs is of great importance. Potentiometric sensors with polymeric plasticized membranes containing neutral ligands are capable of precise, fast, and direct quantification of target analytes. Various ligands with diamide functions have been used for the construction of REM-sensitive sensors. The chemical structures of some of these diamides are very similar to calcium II (ETH 129) and calcium IV (ETH 5234) ionophores suggested in 80’s and successfully commercialized by Fluka Company. Thus, we have hypothesized that these commercial calcium ionophores can provide noticeable potentiometric sensitivity to REM ions. To confirm the validity of this hypothesis, PVC-plasticized sensor membranes based on ETH 129 and ETH 5234 were developed and their potentiometric performance was evaluated in acidic solutions of REM ions and compared to that of well-established neutral ligands, such as tetraoctyldiglycolamide (TODGA). Sensor membranes were synthesized using poly(vinyl chloride) as polymeric matrix, o-nitrophenyloctyl ether (NPOE) as a solvent-plasticizer, ionophore, and chlorinated cobalt dicarbollide (CCD) or fluorinated tetraphenyl borate derivatives (TFPB) as the cation exchanger. Sensors based on calcium ionophores exhibited good sensitivity across the linear concentration range of pC=3 to pC=5 M with the Nernstian slope towards most REM ions at pH 2 and with the detection limit of around 0.1-0.5 mg/l of REMs. The ETH 5234-based sensor demonstrated more remarkable sensitivity compared to the ETH 129-based sensor due to its high lipophilicity. Furthermore, the sensitivity pattern of the sensor based on ETH 5234 looked rather similar to the ligands such as TODGA. The effect of lipophilic CCD and KTFPB cation exchangers on the potentiometric response of the sensors was studied. The results revealed that the sensors containing CCD demonstrated superior sensing characteristics compared to those including TFPB.

In this study, the possibility of using commercial calcium ionophores for the determination of REM ions was presented for the first time. Reproducible and stable results and high sensing performance make them promising alternatives to the expensive ligands such as TODGA for technological monitoring purposes.

Carbonaceous-based nanomaterials (C-NMs) are the pillar of myriad sensing and catalytic electrochemical applications. In this field, the search for environmentally sustainable C-NMs from renewable sources became a duty in the development of nano-sensors. Herein, water-soluble carbon nanofibers (CF) were produced from eucalyptus scraps-based biochar (BH) through an ultrasound treatment, assisted by sodium cholate used as a stabilizing agent. The BH-CF was investigated as sensing material onto commercial screen-printed electrodes via drop-casting (BH-SPE) and as thin-film fully integrated into a lab-made flexible electrode. The thin film was produced via BH-CF vacuum filtration followed by the film transferring to a thermo-adhesive plastic substrate through thermal lamination. This approach gave rise to a conductive BH-CF film (BH-Film) easily embodied in a lab-made electrode produced with office-grade instrumentation (i.e. craft-cutter machine, thermal laminator) and materials (i.e. laminating pouches, stencil). The BH-CF amount was optimized and the resulting film morphologically characterized, then, the electrochemical performances were studied. The BH-CF electrochemical features were investigated towards a broad range of analytes containing phenol moieties, discrimination between orto‐ and mono-phenolic structures were achieved for all the studied compounds. As proof of applicability, the BH-CF-based sensors were challenged for simultaneous determination of mono-phenols and ortho-diphenols in olive oil extracts. LODs ≤ 0.5 μM and ≤ 3.8 μM were obtained for hydroxytyrosol (o-diphenol reference standard) and Tyrosol (m-phenols reference standard), respectively. Moreover, a high inter-sensors precision (RSD calibration-slopes ≤ 7 %, n = 3) and quantitative recoveries in sample analysis (recoveries 91-111%, RSD ≤ 6 %) were obtained. Here, a solvent-free strategy to obtain water-soluble BH-CF was proposed, and their usability to sensor fabrication and modification proved. This work demonstrated as cost-effective and sustainable renewable sources, rationally used, can lead to obtain useful nanomaterials.

Phytochemical products start to be employed to assist 2D nanomaterials exfoliation. However, a lack of studies regarding the molecules involved and their capacity to give rise to functional materials is evident.

In this work, a novel green liquid-phase exfoliation strategy (LPE) is proposed wherein a flavonoid namely catechin (CT) exclusively assists the exfoliation of bulk graphite in conductive water-soluble graphene nanoflakes (GF). Physicochemical and electrochemical methods have been employed to characterize the morphological, structural, and electrochemical features of the GF-CT. Surprisingly, the obtained GF-CT integrates well-defined electroactive quinoid adducts. The resulting few-layers graphene flakes intercalated with CT aromatic skeleton ensure strict electrical contact among graphene sheets, whereas the fully reversible quinoid electrochemistry (ΔE= 28 mV, Ip,a/Ip,c= ⁓1) is attributed to the residual catechol moieties, which work as an electrochemical mediator. The GF-CT intimate electrochemistry is generated directly during the LPE of graphite, not requiring any modification or electro-polymerization steps, resulting in stable (8 months) and reproducible material. The electrocatalytic activity has been proven towards hydrazine (HY) and β-nicotinamide adenine dinucleotide (NADH), a pollutant and a coenzyme, respectively. High sensitivity in extended linear ranges (HY: LOD=0.1 µM, L.R. 0.5-150 µM; NADH: LOD=0.6 µM, L.R. 2.5-200 µM) at low overpotential (+0.15 V) was obtained using amperometry, avoiding electrode-fouling. Improved performances compared with graphite commercial electrodes and graphene exfoliated with a conventional surfactant, were obtained. The GF-CT was successfully used to perform the detection of HY and NADH (recoveries 94-107%, RSD≤8%) in environmental and biological matrices, proving the material exploitability even in challenging analytical applications. On course studies, aim to combine the intrinsic conductivity of the GF-CT with flexible substrates, to construct flexible electrodes/devices able to housing GF-CT-exclusively composed conductive films. In our opinion, the here proposed GF-CT elects itself as a cost-effective and sustainable material, particularly captivating in the (bio)sensoristics scenario.

A highly ordered and oriented graphene doped TiO₂ nanotubes array was synthesised by electrochemical anodization route. High purity graphene oxide suspension was used to prepare from 0.2 wt% graphene oxide (GO) aqueous solution. Then an electrolyte was prepared with 0.5 wt% NH₄F, 10 vol% of GO aqueous solution and ethylene glycol. Two electrodes anodic oxidation was performed for 120 min under 40 V potential where Ti foil was used as the anode and graphite was used as the cathode. Due to the constant availability of GO in the electrolyte, graphene was doped uniformly in the TiO₂ nanotubes without affecting the morphology of TiO₂ nanotubes. Scanning electron micrograph authenticated the formation of highly aligned graphene doped TiO₂ nanotubes with length of 1 µm. X-ray diffraction spectroscopy confirmed the formation of anatase crystallinity in the TiO₂ nanotubes array. The evidence of graphene in the in the hybrid nanotubes was authenticated by the D and G peak in the Raman spectra. X-ray photo electron spectra confirmed the formation of Ti-O-C linkage in the graphene doped TiO₂ nanotube ensuring the formation of high quality composite. Metal-Insulator (oxide)-Metal (MIM) structured sensors were fabricated by using both pure and graphene doped TiO₂ nanotubes. In both the devices, Ti substrate was considered as the bottom electrode and e-beam deposited porous Au was considered as the top electrode. Both the sensors were examined under low concentration methanol. Graphene doped TiO₂ nanotubes depicted a sensitivity of 28% with quite a fast response and recovery time of 34s and 40s towards 100 ppm of methanol. On the other hand, pure TiO₂ nanotubes array depicted a sensitivity of 20% with relatively slow response/recovery time (116s/576s) in the same conditions. A significant improvement in methanol sensing was achieved by the formation of localized heterojunctions between graphene and TiO₂ in the hybrid sample.

Ion sensors with PVC-plasticized membranes are widely employed in routine analytical procedures as a convenient and inexpensive tool for activity assessment. Development of new sensor membrane compositions is a long and tedious process as it requires the search, the synthesis and characterization of potential new membrane active substances. The number of proposed anion-selective ligands for membrane ion sensor is much smaller than that for cations. This is due to the several reasons: large variability of inorganic anion shapes (unlike spherical cations), denser solvate shells of anions, narrow pH range of some forms. The particular problem relates to the development of the sensor with selectivity towards hydrophilic anions, like, e.g. carbonate. An attractive opportunity would be to create a model that could predict the selectivity of ligand towards carbonate without real synthesis and characterisation of the corresponding sensors. An appropriate tool for that can be QSPR (Quantitative Structure-Property Relationship) approach [1], where the chemical structure of organic ligand is encoded with a set of formal descriptors and related through the mathematical modelling to the target property (selectivity in our case).

In this work, based on 48 ligand structures suggested in literature for carbonate sensing we have developed a model for prediction of logK (HCO₃^-/Cl^-). Substructural Molecular Fragments (SMF) were used to describe the structure of compounds, where fragments were considered as sequences of bonds and atoms. The Projection on Latent Structures (PLS) method was used to calculate the regression model. The obtained model was tested in prediction of selectivity of several newly synthesized ligands. The details on the results will be provided in the presenstation.

This study was supported by RFBR project #20-33-70272.

References:

1. Katritzky A.R., Lobanov V.S. and Karelson M. Soc. Rev., 1995, 24, 279-287.

Low-cost gas sensors detect pollutants gas at part per billion level and may be installed in small devices to densify air quality monitoring networks and to analyze the spread of pollutants around an emissive source. However, these sensors suffer from several issues such as environmental factors impact and cross-interfering gases. For instance, ozone (O₃) electrochemical sensor senses nitrogen dioxide (NO₂) and O₃ simultaneously without discrimination. Alphasense proposes the use of pair of sensors, the first one is equipped with filter dedicated to measure NO₂. The second one is sensitive to both NO₂ and O₃. Thus, O₃ concentration can be obtained by subtracting the concentration of NO₂ from the sum of the two concentrations. This technic is not practical and requires calibrating each sensor individually leading to biased concentration estimation. In this paper we propose partial least square regression (PLS) to build a calibration model including both of sensors responses and temperature and humidity variations. The results obtained from data collected on field for two months show that PLS regression provides better gases concentrations estimation in terms of accuracy than calibrating each sensor individually.

Currently, control of the quality and composition of consumed drinking water has become extremely popular. One of its important parameters is the content of iron ions, an excess of which causes toxic effects. The detection of low concentrations of iron ions in drinking water requires highly sensitive and fast methods with the possibility of mass application by a wide range of consumers.

This work demonstrates the use of monodispersed gold nanoparticles (AuNP) coated with mercaptosuccinic acid (MSA) as a reagent for the determination of Fe³⁺ ions. Mercaptosuccinic acid as a thiol compound provides reduction and stabilization in the synthesis of gold nanoparticles. In addition, the presence of two carboxyl groups makes it possible to bind Fe³⁺ ions. In the proposed method, the binding of Fe³⁺ ions by AuNP-MSA complexes causes aggregation in a colloidal solution, accompanied by recorded spectral changes.

The synthesize AuNP-MSA preparations were characterized by transmission electron microscopy and dynamic light scattering; the stated average particle diameter is 19.8 nm. The absorption ratios of A₅₃₀/A₆₅₀ was selected as an analytical signal and corresponded to a change in the color of the solution from red to blue. The effect of the pH of the system on changes in the analytical signal in the presence of Fe³⁺ ions was studied; the maximum detection sensitivity was achieved at pH 4.5. The optimal ratio of the volumes of the AuNP-MSA colloidal solution (with the optical density around 1.0) and the volumes the test sample was established, equal to 1:30. The proposed approach provides reliable detection of Fe³⁺ ions in concentrations up to 10 ng/ml, which is 30 times lower than the maximum allowable concentration for drinking water. The analytical system is also highly selective; samples containing other heavy metal ions did not cause significant optical changes. To confirm the practical application of the developed system, samples of drinking, spring and tap water with different contents of Fe³⁺ ions were analyzed.

The obtained results indicate that the use of gold nanoparticles functionalized with mercaptosuccinic acid as an analytical reagent is promising. The developed technique makes it possible to determine ultra-low concentrations of Fe³⁺ ions quickly and without using expensive equipment.

This work was supported by the Russian Science Foundation (Grant No. 19-44-02020) and Ministry of Science and High Education.