Introduction

Honey is a widely consumed product globally, so it is interesting to develop rapid and inexpensive methods for its authentication and quality control. On this line, two of the most studied parameters are total phenolic content (TPC) and antioxidant activity (AOX) [1].

The most common methods to analyze both parameters are spectroscopic assays using the Folin-Ciocalteu reagent (TPC) and 2,2′-azino-bis(3-ethylbenzothiazoline-6 sulfonic acid) diammonium salt (ABTS) as a radical source (TEAC) [2]. These methods require a large expenditure of money and time and specialized personnel.

This study's main objective is to develop a suitable method that allows us to quantify the total phenolic content and determine the antioxidant activity in a faster and cheaper way than the conventional methods. To achieve this objective, a chromogenic sensor has been developed for the rapid and low-cost determination of the both parameters mentioned above in a single measurement. In addition to being a faster and cheaper method, as it is a polymeric sensor, it has advantages of lack of migration of the sensor subunits, manageability, and possibility of working in solid-state.

Experimental

This method is based on hydrophilic colorimetric films with pendant benzenediazonium salt motifs, which react with phenols rendering highly colored diazo groups. The preparation of the starting material is based on vinylpyrrolidone (VP), methyl methacrylate (MMA) and 4-amino styrene (SH₂). The benzenediazonium salt was formed from aniline pendant groups by immersion in an aqueous acid solution of sodium nitrite [3]. The intensity of the color allows us to determine both TPC and TEAC of the sample by analyzing a picture taken with a smartphone analyzed using the color-definition-parameters (RGB).

For the analysis with the honey samples, our film's 8 mm diameter discs were dipped in 10 ml of each honey sample for 2 h. After that time, the discs were removed from the sample and washed 3 times with NaOH 0.1M. Finally, photographs of the discs were taken, RGB parameters were analysed, and were compared with the antioxidant activity results and total polyphenol content obtained by conventional methods mentioned above.

Conclusions

We have developed a new method to quantify the total polyphenol content and determine the antioxidant activity with a single analysis in all honey samples studied. This method reduces the time and the cost of the analysis and does not require trained personnel, so it has great potential in the quality control of honey samples.

Acknowledgment

We gratefully acknowledge the financial support provided by FEDER (Fondo Europeo de Desarrollo Regional), and both the Spanish Ministerio de Economía, Industria y Competitividad (MAT2017-84501-R) and the Consejería de Educación—Junta de Castilla y León (BU061U16 and BU041G18) are gratefully acknowledged.

References

1. Thrasyvoulou, A., Tananaki, C., Goras, G., Karazafiris, E., Dimou, M., Liolios, V., Gounari, S. (2018). Span. J. Agric. Res, 57(1), 88-96.
2. Re, R., Pellegrini, N., Proteggente, A., Pannala, A., Yang, M., Rice-Evans, C. (1999). Free Biol. Med. 26(9-10), 1231-1237
3. Vallejos, S., Moreno, D., Ibeas, S., Muñoz, A., García, F. C., García, J. M. (2019). Food Control. 106, 106684.

Sexually transmitted diseases (STDs) refer to infections and syndromes caused by bacteria, viruses, protozoa. It is estimated that each year around 214 million people struggle with STDs caused only by bacteria: Chlamydia trachomatis (127 million) and Neisseria gonorrhoeae (87 million). That makes STDs an epidemic and can lead to numerous economic and health consequences. There are several methods that enable the diagnosis of STDs, but each of them has some limitations e.g.: Gram staining is characterized by its low detection rate while microbial culture requires time-consuming incubation and specific conditions for bacterial growth. Even the most recommended tests - nucleic acid amplification tests (NAATs) are very expensive and not every laboratory can afford them. For the above-described reasons, there is still a need to establish a rapid, reliable, and sensitive method for STDs diagnosis.

More recently, a lot of studies have been done presenting the great potential of the application of SERS (Surface-enhanced Raman Spectroscopy) in diverse fields including medicine and biology. SERS is a kind of fingerprint technique based on the inelastic scattering of incident light by molecules adsorbed on the roughened metal surface (SERS-active substrate). The phenomenon of the SERS technique originates mainly from two main mechanisms: electromagnetic (EM) and chemical.

In this study, we present that SERS-based sensor and chemometric analysis can be performed successfully in a direct and indirect manner for STD diagnosis. The indirect (confirmatory) approach is based on the identification of unknown pathogenic strain in the clinical samples, by comparison, its spectral image to other spectral images of different bacteria. While the direct one guarantees ultrafast diagnosis (up to 15 min) by classifying SERS spectra of clinical sample to the correct group by means of supervised technique (SIMCA, PLS1-DA). The undoubted advantage of this approach is simplified procedure while maintaining ultra-high sensitivity. Hence, both of these methods can compete with many currently used techniques.

This research may have a great impact on the biomedical applications since, the integration of SERS-based sensors with a small, portable Raman spectrometer could lead to the development of a handheld point-of-care device, which would enable the diagnosis of STD in an extremely short time [1].

[1] S. Berus et al. Patent Application P.436251, 2021

The analytical methods used to determine germanium must be very sensitive as the germanium content in natural materials and waste rarely exceeds the mg/kg level. Among the numerous analytical techniques available, catalytic adsorptive stripping voltammetry (CAdSV) plays an essential role due to its remarkable sensitivity. After careful selection of the catalytic reagent and electrode material, the detection limits at sub-nM level are feasible. In the present work, it has been shown that the properties of lead and bismuth plated electrodes (plating solutions: quiescent 0.34 M HClO₄ containing 0.043 M of Bi(III) or stirred 0.2 M acetate buffer containing 0.003M Pb(II), E_plat = -0.9 V, Q_plat = 0.8 mC per mm²) differ considerably and only bismuth film electrodes enable germanium analytical signals to be obtained when Ge(IV)-catechol-V(IV)-HEDTA (HEDTA - N-hydroxyethyl-ethylene diamine-triacetic acid) catalytic system is employed. The bismuth film electrodes deposited on screen-printed carbon (Bi/SPE) supports provided well-shaped, sensitive, and reproducible signals of Ge(IV) (RSD = 2%) in the supporting electrolyte containing 0.05 M acetate buffer (pH of 4.4), 1 mM of catechol, 1 mM of V(IV) and 1.5 mM of HEDTA. The calibration curve was linear within the range from 2 to 30 nM of Ge(IV) (LOD = 1.5 nM). The applicability of the Bi/SPE electrodes was verified by measuring Ge(IV) in spiked snow water samples. The concentration determined by the standard addition method was equal to 10.05±0.11 nM of Ge(IV) that correlates well with the spiked value equal to 10 nM of Ge(IV).

Hydrogen sulfide (H₂S) is a highly toxic and dangerous compound, capable of causing severe health problems after prolonged exposure even at low concentrations. It is a gas, slightly solvable in water at acid pH. Nonetheless, as pH increases its labile protons are lost and it becomes the more soluble HS^- ion. It appears in wastewater treatment plants and gas treatment bio-scrubbers and is still highly pollutant and hazardous. Thus, it is of critical importance to develop effective methods to monitoring H₂S in biological treatment process and water systems.

Inkjet printing technology has been proven in the recent years as an economic, fast, reproducible and highly versatile method of mass-producing micro-electrodes. Those can themselves be made of a large variety of materials, from metals to polymers. Tuned with the appropriate transductors, many electrodes can become sensors for analytes of interest. Considering the hazards produced by chemicals like H₂S, miniaturized systems like this are becoming the new sensing platforms for tracking pollutants.

Herein, an easy to produce, low-cost, miniaturized and inkjet-printed amperometric H₂S sensor is presented. A gold electrode, coupled to a conductive track of silver, is modified with a mixture of Single-Walled Carbon Nanotubes (SWCNTs), Poly(VinylAlcohol) (PVA) and Poly(DiallylDimethylAmmonium Chloride) (PDDA). It detects HS^- by oxidizing it into elemental sulfur (S⁰), recording the produced current from this reaction. It has an effective working pH range of 6.5-13 and a wide linear range response from 6 µM to 592 µM of HS^-. Tests show that the sensor is also capable of working on complex samples, such as reactor media.

Following the great success of graphene, 2D layered materials based on the elements of group VA (also known as pnictogens) open up many possibilities in the field of sensors. This group of materials include phosphorene, bismuthene, antimonene, and arsenene and offer many desirable features for electrochemical sensors such as high surface area, excellent mobility, morphology tunability, and the possibility to modify their surface properties [1].

In this work, the modification of screen-printed electrodes with 2D layered pnictogens was explored for the enhanced anodic stripping voltammetric determination of metal ions. Particular emphasis was placed on bismuthene and antimonene given both their lower toxicity and the ability of bismuth and antimony films to mirror the analytical performance of mercury electrodes for metal ion determination. Thus, bismuthene and antimonene, as well as some of their derivatives, were tested and compared looking for an improved analytical performance (i.e, low limit of detection, LOD, high sensitivity), which was evaluated for the simultaneous determination of Pb(II) and Cd(II). Out of all the tested materials, bismuthene demonstrated the best analytical performance, providing, for a 120 s preconcentration time, a linear response from 0.2 to 25.0 μg L⁻¹ for both Pb(II) and Cd(II) and LODs of 0.06 and 0.07 μg L^-1 for Pb(II) and Cd(II), respectively [2]. The achieved LODs also represent an improvement over other bismuth-based electrochemical sensors such as those based on bismuth nanoparticles or commercially available sputtered screen-printed electrodes.

[1] M.A. Tapia, R. Gusmão, N. Serrano, Z. Sofer, C. Ariño, J.M. Díaz-Cruz, et al., Phosphorene and other layered pnictogens as a new source of 2D materials for electrochemical sensors, TrAC Trends Anal. Chem. 139 (2021) 116249. doi:10.1016/j.trac.2021.116249.

[2] M.A. Tapia, C. Pérez-Ràfols, R. Gusmão, N. Serrano, Z. Sofer, J.M. Díaz-Cruz, Enhanced voltammetric determination of metal ions by using a bismuthene-modified screen-printed electrode, Electrochim. Acta. 362 (2020) 137144. doi:10.1016/j.electacta.2020.137144.