Gold Nanoparticles-based Extraction-Free Colorimetric Assay in Organic Media: An Optical Index for Determination of Total Polyphenols in Fat-Rich Samples

Rapid pretreatment-free evaluation of antioxidant capacity in extra virgin olive oil using a laser-nanodecorated electrochemical lab-on-strip

An electroanalytical lab-on-a-strip device for the direct extra-virgin olive oil (EVOO) antioxidant capacity evaluation is proposed. The lab-made device is composed of a CO₂ laser nanodecorated sensor combined with a cutter-plotter molded paper-strip designed for EVOOs sampling and extraction. Satisfactory performance towards the most representative o-diphenols of EVOOs i.e., hydroxytyrosol (HY) and oleuropein (OL) were achieved; good sensitivity (LOD_HY = 2 µM; LOD_OL = 0.6 µM), extended linear ranges (HY: 10-250 µM; OL: 2.5-50 µM) and outstanding reproducibility (RSD < 5%, n = 3) were obtained in rectified oil. The device was challenged for the extraction-free analysis of 15 different EVOO samples, with satisfactory recoveries (90-94%; RSD < 5%, n = 3) and correlation with classical photometric assays (r ≥ 0.91). The proposed device includes all analysis steps, needs 4 µL of sample, and returns reliable results in 2 min, resulting portable and usable with a smartphone.

Metal nanoparticles based lab-on-paper for phenolic compounds evaluation with no sample pretreatment. Application to extra virgin olive oil samples

In this work, a low-cost, disposable, and portable lab-on-paper device is proposed to simultaneously quantify total polyphenol content (TPC) and antioxidant capacity (AOC) in 15 min; the assay requires no pre-treatment of the samples. The lab-on-paper device fabrication has been carried out employing a xurography-based benchtop microfabrication technology using low-cost materials as chromatography paper and polymeric sheets. Extra virgin olive oil (EVOO) phenolic compounds' represents a nutritional added value, nevertheless, the high lipidic content hinders their direct and rapid analysis, resulting in an extremely challenging sample. The realized lab-on-paper allows to perform the dual TPC and AOC determination in three simple steps: (i) sample loading, (ii) analytes transport to the analysis spot, and (iii) double colorimetric analysis exploiting the growth of AuNPs and AgNPs on paper mediated by phenolic compounds. Signal acquisition is achieved using a standard digital camera. The dual colorimetric assay is able to detect phenolic compounds in the 25-500 mg L^-1 range with limits of detection ≤6 mg L^-1 and good reproducibility (RSDs ≤11%). Direct analysis of EVOO samples (n = 30) correlated well (r > 0.92) with conventional spectrophotometric methods for TPC and AOC determination.

A Smartphone-Based Chemosensor to Evaluate Antioxidants in Agri-Food Matrices by In Situ AuNP Formation

In recent years, there has been a continuously growing interest in antioxidants by both customers and food industry. The beneficial health effects of antioxidants led to their widespread use in fortified functional foods, as dietary supplements and as preservatives. A variety of analytical methods are available to evaluate the total antioxidant capacity (TAC) of food extracts and beverages. However, most of them are expensive, time-consuming, and require laboratory instrumentation. Therefore, simple, cheap, and fast portable sensors for point-of-need measurement of antioxidants in food samples are needed. Here, we describe a smartphone-based chemosensor for on-site assessment of TAC of aqueous matrices, relying on the antioxidant-induced formation of gold nanoparticles. The reaction takes place in ready-to-use analytical cartridges containing an hydrogel reaction medium preloaded with Au(III) and is monitored by using the smartphone's CMOS camera. An analytical device including an LED-based lighting system was developed to ensure uniform and reproducible illumination of the analytical cartridge. The chemosensor permitted rapid TAC measurements of aqueous samples, including teas, herbal infusions, beverages, and extra virgin olive oil extracts, providing results that correlated with those of the reference methods for TAC assessment, e.g., oxygen radical absorbance capacity (ORAC).

Designing of Nanomaterials-Based Enzymatic Biosensors: Synthesis, Properties, and Applications

Among the many biological entities employed in the development of biosensors, enzymes have attracted the most attention. Nanotechnology has been fostering excellent prospects in the development of enzymatic biosensors, since enzyme immobilization onto conductive nanostructures can improve characteristics that are crucial in biosensor transduction, such as surface-to-volume ratio, signal response, selectivity, sensitivity, conductivity, and biocatalytic activity, among others. These and other advantages of nanomaterial-based enzymatic biosensors are discussed in this work via the compilation of several reports on their applications in different industrial segments. To provide detailed insights into the state of the art of this technology, all the relevant concepts around the topic are discussed, including the properties of enzymes, the mechanisms involved in their immobilization, and the application of different enzyme-derived biosensors and nanomaterials. Finally, there is a discussion around the pressing challenges in this technology, which will be useful for guiding the development of future research in the area.

An Overview of Optical and Electrochemical Sensors and Biosensors for Analysis of Antioxidants in Food during the Last 5 Years

Antioxidants are a group of healthy substances which are useful to human health because of their antihistaminic, anticancer, anti-inflammatory activity and inhibitory effect on the formation and the actions of reactive oxygen species. Generally, they are phenolic complexes present in plant-derived foods. Due to the valuable nutritional role of these mixtures, analysis and determining their amount in food is of particular importance. In recent years, many attempts have been made to supply uncomplicated, rapid, economical and user-friendly analytical approaches for the on-site detection and antioxidant capacity (AOC) determination of food antioxidants. In this regards, sensors and biosensors are regarded as favorable tools for antioxidant analysis because of their special features like high sensitivity, rapid detection time, ease of use, and ease of miniaturization. In this review, current five-year progresses in different types of optical and electrochemical sensors/biosensors for the analysis of antioxidants in foods are discussed and evaluated well. Moreover, advantages, limitations, and the potential for practical applications of each type of sensors/biosensors have been discussed. This review aims to prove how sensors/biosensors represent reliable alternatives to conventional methods for antioxidant analysis.

Effect of phenolic compounds-capped AgNPs on growth inhibition of Aspergillus niger

An exponential increase of scientific works dealing with the use of polyphenol-rich 'natural products' for the synthesis of bioactive AgNPs is in progress. However, a lack of fundamental studies on phytochemical compounds involved, and their role is evident. In this work, a comprehensive study of the antifungal performances of silver nanoparticles (AgNPs) synthesized exclusively with phenolic compounds (PCs) with different structures and different antioxidant capacity is presented. The experimental hypothesis is that AgNPs@PCs produced with different PCs can exert different toxicity. In particular, di-hydroxylic and tri-hydroxylic phenolic acids (caffeic acid and gallic acid) and flavonoids (catechin and myricetin) were compared. A room temperature rapid and simple AgNPs synthesis was carefully optimized, obtaining stable and reproducible colloids. AgNPs@PCs suspensions were characterized by UV-vis spectroscopy, ς-potential, dynamic light scattering and transmission electron microscopy. AgNPs@PCs radical scavenging capacity was also assessed. Finally, the AgNPs@PCs antifungal effect was tested against Aspergillus niger, particularly on spore germination and mycelial growth. The different antifungal activity was attributed to the different PCs' ability to generate/stabilize AgNPs with different shells, residual antioxidant capacity, and capacity to interact and aggregate during their 'attack' to A. niger hyphae. This work paves the way for the rational use of PCs and PCs rich-products for AgNPs-based applications.

Colorimetric sensors and nanoprobes for characterizing antioxidant and energetic substances

The development of analytical techniques for antioxidant compounds is important, because antioxidants that can inactivate reactive species and radicals are health-beneficial compounds, also used in the preservation of food and protection of almost every kind of organic substance from oxidation. Energetic substances include explosives, pyrotechnics, propellants and fuels, and their determination at bulk/trace levels is important for the safety and well-being of modern societies exposed to various security threats. Most of the time, in field/on site detection of these important analytes necessitates the use of colorimetric sensors and probes enabling naked-eye detection, or low-cost and easy-to-use fluorometric sensors. The use of nanosensors brings important advantages to this field of analytical chemistry due to their various physico-chemical advantages of increased surface area, surface plasmon resonance absorption of noble metal nanoparticles, and superior enzyme-mimic catalytic properties. Thus, this critical review focuses on the design strategies for colorimetric sensors and nanoprobes in characterizing antioxidant and energetic substances. In this regard, the main themes and properties in optical sensor design are defined and classified. Nanomaterial-based optical sensors/probes are discussed with respect to their mechanisms of operation, namely formation and growth of noble metal nanoparticles, their aggregation and disaggregation, displacement of active constituents by complexation or electrostatic interaction, miscellaneous mechanisms, and the choice of metallic oxide nanoparticles taking part in such formulations.

Plasmonic active film integrating gold/silver nanostructures for H2O2 readout

A nanostructured Ag/Au adhesive film for H₂O₂ reagentless determination is here proposed. The film has been realised onto ELISA polystyrene microplates. Microwells surface has been initially modified with a gold nanoparticles (AuNPs)/polydopamine thin-film. The pristine AuNPs-decorated film was later functionalized with catechin (Au-CT) allowing a uniform formation of a plasmonic active nanostructured silver network in presence of Ag⁺. Changes in localized surface plasmon resonance (LSPR) of the silver network upon addition of H₂O₂ has been used as analytical signal, taking advantage of the etching phenomenon. The Ag/Au nanocomposite-film is characterized by a well-defined (LSPR_max = 405 ± 5 nm), reproducible (intraplate RSD ≤ 9.8%, n = 96; inter-plate RSD ≤ 11.4%, n = 480) and stable LSPR signal. The film's analytical features have been tested for H₂O₂ and glucose (bio)sensing. Satisfactory analytical performances were obtained both for H₂O₂ (linear range 1-200 μM, R² = 0.9992, RSD ≤ 6.3%, LOD = 0.2 μM) and glucose (linear range 2-250 μM, R² = 0.9998, RSD ≤ 8.9%, LOD = 0.4 μM). As proof of applicability, the determination of the two analytes in soft drinks has been carried out achieving good and reproducible recoveries (84-111%; RSD ≤ 9%). The developed nanostructured film overcomes analytical drawbacks associated with the use of colloidal dispersions in plasmonic assays carried out in solution; the low cost, robustness, ease of use and possibility of coupling enzymatic reactions appears very promising for (bio)sensors based on the detection of H₂O₂.

Colorimetric discriminatory array for detection and discrimination of antioxidants based on HAuCl4/3,3',5,5'-tetramethylbenzidine

Here, we report a simple but effective nose/tongue-mimic sensor array based on HAuCl₄/3,3',5,5'-tetramethylbenzidine (TMB) for colorimetric discrimination and determination of antioxidants. Two concentrations of HAuCl₄ were employed as receptor units to construct the colorimetric sensor array. The sensing strategy is based on the fact that HAuCl₄ with different concentrations (0.08 and 0.03 mM) could oxidize TMB to oxidized TMB (oxTMB), resulting in a blue and green color solution, respectively, corresponding to an absorption peak centered at 440 nm and 657 nm. However, the presence of different antioxidants could cause the reduction in HAuCl₄, leading to the fading of the blue and green color and the decrease in the absorbance at 440 nm and 657 nm to varying degrees. Based on the above phenomena, by taking advantage of linear discriminant analysis (LDA), five antioxidants (i.e. ascorbic acid (AA), melatonin (MT), uric acid (UA), tannic acid (TCA), and glycine (Gly)) at five concentrations (200, 120, 60, 20, and 1 nM) were successfully discriminated both in buffer and serum. More importantly, this approach is simple, fast, and without the use of any nanomaterials.

Class-selective voltammetric determination of hydroxycinnamic acids structural analogs using a WS2/catechin-capped AuNPs/carbon black-based nanocomposite sensor

A high-performance screen-printed electrode (SPE) based nanocomposite sensor integrating tungsten disulfide (WS₂) flakes decorated with catechin-capped gold nanoparticles (AuNP-CT) and carbon black (CB) has been developed. The excellent antifouling properties of WS₂ decorated with AuNP-CT into a high conductivity network of CB results in high selectivity, sensitivity, and reproducibility for the simultaneous determination of hydroxycinnamic acid (hCN) structural analogs: caffeic (CF), sinapic (SP), and p-coumaric acids (CM). Using differential pulse voltammetry (DPV), the target hCNs resulted in three well-resolved oxidation peaks at SPE-CB-WS₂/AuNP-CT sensor. Excellent antifouling performance (RSD i_p,a ≤ 3%, n = 15 for three analytes' simultaneous measure) and low detection limits (CF 0.10 μmol L^-1; SP, 0.40 μmol L^-1; CM, 0.40 μmol L^-1) are obtained despite the analyzed compounds having a high passivation tendency towards carbon-based sensors. The SPE-CB-WS₂/AuNP-CT sensor was successfully applied to determine CF, SP, and CM in food samples with good precision (RSD ≤ 4%, n = 3) and recoveries (86-109%; RSD ≤ 5%, n = 3). The proposed sensor is the first example exploiting the simultaneous determination of these compounds in food samples. Given its excellent electrochemical performance, low cost, disposability, and ease of use, this SPE-CB-WS₂/AuNP-CT nanocomposite sensor represents a powerful candidate for the realization of electrochemical devices for the determination of (bio)compounds with high passivation tendency. Graphical abstract.

Colorimetric sensor array for accurate detection and identification of antioxidants based on metal ions as sensor receptors

There is an ongoing need to develop high-performance sensing strategy for detecting and discriminating antioxidants, primarily because of their role in medical diagnosis and food. In this regard, visual sensor arrays have been a subject of intensive research for such applications. To this end, we propose a colorimetric sensor array for accurate detection and identification of antioxidants, which is based on the reactions between 3,3',5,5'-tetramethylbenzidine (TMB) and metal ions as sensing receptors and the interactions between antioxidants and oxidized TMB (oxTMB). Different target antioxidants displayed diverse reduction abilities toward the oxTMB, creating distinct colorimetric response patterns. The combination of colorimetric response variation at color and absorbance at 652 nm enables the sensor array to provide a unique fingerprint pattern to each antioxidant. Linear discriminant analysis (LDA) and centroid diagrams show that the sensor array can well detect and discriminate the eight tested antioxidants, including lipoic acid (LIA), cysteine (Cys), tannin (TA), ascorbic acid (AA), glutathione (GSH), Uric Acid (UA), glycine (Gly), and dopamine (DA), with a high sensitivity in the range of nanomolar concentrations.

Metal-Organic Framework Loaded by Rhodamine B As a Novel Chemiluminescence System for the Paper-Based Analytical Devices and Its Application for Total Phenolic Content Determination in Food Samples

Herein, a novel paper-based chemiluminescence (CL) device has been reported for the estimation of total phenolic content of food samples. The CL system implemented on the paper was based on a hydrogen peroxide (H₂O₂)-rhodamine b (RhoB)-cobalt metal organic framework (CoMOF) reaction. It was found that the reaction of H₂O₂ with RhoB molecules, loaded into the nanopores of CoMOF (R@CoMOF), can produce an intensive CL emission. The experiments on the paper indicated that in the presence of CoMOF, the CL emission was greatly increased. In addition to this strong catalyzing effect, application of CoMOF on the paper improved the stability of the CL system for several days. As a useful analytical application for the obtained paper-based CL device (PCD), it was examined for the detection of phenolic antioxidants. It was observed that the addition of 5 μL of phenolic compounds (PC) on the paper containing the CL reagents can remarkably decrease the CL intensity. This effect was applied to design a simple analytical assay for PC. After the optimization process, the best sensitivity was obtained for gallic acid, quercetin, catechin, kaempferol, and caffeic acid with detection limits of 0.98, 1.36, 1.48, 1.81, and 2.55 ng mL^-1, respectively. The relative standard deviations (RSD%) were also less than 5%. This study is the first report on the practical application of PCD using a nanomaterial assisted CL reaction. It is simple, portable, and low-cost and consumes a very low amount of reagents and sample solution. The device was successfully applied in the investigation of total antioxidant capacity of molasses and honey samples.

Nanohybrid carbon black-molybdenum disulfide transducers for preconcentration-free voltammetric detection of the olive oil o-diphenols hydroxytyrosol and oleuropein

A new hybrid nanomaterial is used in a screen-printed electrode (SPE) for sensing of the ortho-diphenols oleuropein (OLEU) and hydroxytyrosol (HYT) in extra virgin olive oil (EVOO) and related samples. The hybrid material consists of carbon black (CB) and molybdenum disulfide (MoS₂). In comparison with individual nanomaterials, CB-MoS₂ exhibits improved charge-transfer ability, low charge-transfer resistance, high electrical conductivity and enhanced electrocatalysis. The sensor is also characterized by (a) high sensitivity that avoids the need for adsorptive voltammetry, (b) reduced analysis time, and (c) high anti-fouling ability (electrode RSD_OLEU < 8%, for n = 10). OLEU can be detected in the 0.3 to 30 μM concentration range with a 0.1 μM LOD, and HYT in the 2-100 μM range with a 1 μM LOD. A comparison of the data obtained by this sensor and by HPLC-UV exhibited high correlation (r = 0.995, p < 0.05). These data revealed the reliability of CB-MoS₂ for analysis of complex EVOO and related samples. Graphical abstract CB-MoS₂-based electrochemical sensor for fast and reliable assessment of total ortho-diphenols antioxidants in olive oils.

Colorimetric Sensor Arrays for Antioxidant Discrimination Based on the Inhibition of the Oxidation Reaction between 3,3',5,5'-Tetramethylbenzidine and Hydrogen Peroxides

The discrimination of antioxidants is of great significance because of their essential roles in various biological processes and many diseases. Compared with the traditional lock-key sensing mode for single target detection at a time, sensor arrays can discriminate various antioxidants simultaneously. Nanomaterial-based sensor arrays have shown great promise for antioxidant discrimination; however, as far as it is known, none of them have been reported for discriminating antioxidants based on the catalytic reaction of intrinsic peroxidase-like activity of two-dimensional nanomaterials. To fill the gap, we herein unveil a colorimetric (e.g., UV-vis absorption) approach for antioxidant discrimination based on the three nanomaterial [graphene oxide, molybdenum disulfide (MoS₂), and tungsten disulfide (WS₂)]-catalyzed 3,3',5,5'-tetramethylbenzidine (TMB)-hydrogen peroxide (H₂O₂) reaction system. In this sensor array, the antioxidants inhibit the reaction between TMB and H₂O₂, resulting in different colorimetric response patterns. The obtained patterns for five antioxidants, including ascorbic acid, cysteine, melatonin, uric acid, and glutathione (GSH), at the 60 nM level, were successfully discriminated using linear discriminant analysis both in buffer and serum samples.

Noble Metal Nanoparticles Applications: Recent Trends in Food Control

Scientific research in the nanomaterials field is constantly evolving, making it possible to develop new materials and above all to find new applications. Therefore, nanoparticles (NPs) are suitable for different applications: nanomedicine, drug delivery, sensors, optoelectronics and food control. This review explores the recent trend in food control of using noble metallic nanoparticles as determination tools. Two major uses of NPs in food control have been found: the determination of contaminants and bioactive compounds. Applications were found for the determination of mycotoxins, pesticides, drug residues, allergens, probable carcinogenic compounds, bacteria, amino acids, gluten and antioxidants. The new developed methods are competitive for their use in food control, demonstrated by their validation and application to real samples.

Smartphone-based colorimetric assay of antioxidants in red wine using oxidase-mimic MnO2 nanosheets

A smartphone-based colorimetric method is developed for the determination of total antioxidants in red wine using oxidase-mimic MnO₂ nanosheets as probes.

Application of Gold-Nanoparticle Colorimetric Sensing to Rapid Food Safety Screening

Due to their unique optical properties, narrow size distributions, and good biological affinity, gold nanoparticles have been widely applied in sensing analysis, catalytic, environmental monitoring, and disease therapy. The color of a gold nanoparticle solution and its maximum characteristic absorption wavelength will change with the particle size and inter-particle spacing. These properties are often used in the detection of hazardous chemicals, such as pesticide residues, heavy metals, banned additives, and biotoxins, in food. Because the gold nanoparticles-colorimetric sensing strategy is simple, quick, and sensitive, this method has extensive applications in real-time on-site monitoring and rapid testing of food quality and safety. Herein, we review the preparation methods, functional modification, photochemical properties, and applications of gold nanoparticle sensors in rapid testing. In addition, we elaborate on the colorimetric sensing mechanisms. Finally, we discuss the advantages and disadvantages of colorimetric sensors based on gold nanoparticles, and directions for future development.

Lab-on-a-micromotor: catalytic Janus particles as mobile microreactors for tailored synthesis of nanoparticles

Catalytic Janus micromotors encapsulating Cd2+ or citrate are used here as mobile microreactors for "on the fly" CdS quantum dot and gold nanoparticle synthesis. Micromotor navigation in microliter "reagent solutions" results in the generation of the corresponding nanoparticles inside the micromotor body with high yield and negligible waste generation. Nanoparticle generation can be attributed to convective diffusion of reagents into the moving reactor body. "On-demand" modulation of nanoparticle size and catalytic activities can be achieved by judicious control of the motion behavior of the microreactor. The use of confined reagents in connection with such enhanced movement allows for efficient operation in very low (less than 800 μL) volumes. The new microreactors developed here hold considerable promise for reactions in aqueous environments for novel synthetic schemes in different sites along with multiplexed capabilities for a myriad of catalytic reactions.

Silver nanoparticles-based plasmonic assay for the determination of sugar content in food matrices

A simple colorimetric assay for sugars (monosaccharides, polyols and disaccharides) quantification based on silver nanoparticles (AgNPs) formation is proposed. Sugars are able to form AgNPs reducing Ag⁺ and stabilizing the suspension. Since the driving force is not only chemical reduction, the optimized assay allows the determination of both "reducing" and "non-reducing" sugars with similar reactivity. The localized surface plasmon resonance (LSPR) absorption band with maximum wavelength centered at ≈ 430 nm (A₄₃₀) is used for the detection. Monosaccharides, disaccharides and polyols with different functional groups and structure have been investigated: D-(+)-glucose, D-(+)-galactose, D-(-)-fructose, sucrose, D-(+)-raffinose D-(+)-maltose, D-(+)-trehalose, D-mannitol, D-sorbitol, i-inositol, xylitol and D-(+)-xylose. The resulting AgNPs have been characterized by UV-Vis spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM). The reactivity of monomers and polyols was found higher than the disaccharides. The AgNPs-based method was applied to the determination of the sugars content in soft drinks (n = 6) and apple extracts (n = 5). Sugars content (expressed as glucose equivalents) determined by the proposed assay resulted comparable to results obtained by ion chromatography (R = 0.994). Reproducibility (RSD ≤ 9.4%) and recovery values (from 86.1% to 117.7%) in food matrices were satisfactory. Moreover, the assay is selective vs. potentially interfering compounds found in food. Considering the assay time (10 min), the low cost of reagents, limited volumes of sample (5-100 μL), no use of organic solvents and low waste generation, this assay appears very promising for sugars content determination.

An optoelectronic tongue based on an array of gold and silver nanoparticles for analysis of natural, synthetic and biological antioxidants

A colorimetric array, which can discriminate 20 food antioxidants of natural, synthetic and biological groups, is described. It consists of gold and silver nanoparticles that were synthesized using six different reducing and/or capping agents. The function of the array relies on the interaction of the antioxidants with the nanoparticles which causes aggregation or morphological changes. This, in turn, causes a change in the sensors' colors. The array produces a unique combination of colors for each antioxidant. The resulting colorations are determined by recording the absorbances of the arrays at wavelengths of 405, 450, 490 and 630 nm, or by capturing the images with a digital camera. The discriminatory ability of the array is investigated by principle component analysis and hierarchical cluster analysis. The method was applied to quantitative assay of gallic acid, caffeic acid, catechin, dopamine, citric acid, butylated hydroxytoluene and ascorbic acid. The respective limits of detection are 4.2, 13, 53, 6.9, 47, 3.5 and 43 nM, respectively. The simultaneous determination of 5 different antioxidants is achieved utilizing partial least square regression. The root mean square errors for prediction of the test set are 0.0650, 0.0782, 0.811, 0.0206 and 0.135 nM for gallic acid, catechin, butylated hydroxytoluene, dopamine, and ascorbic acid, respectively. This method demonstrates excellent potential for analysis of antioxidants in beverages such as tea and lemon juice. Graphical abstract Schematic of a method for the simultaneous determination of several antioxidants based on changes in the color of gold and silver nanoparticles. The antioxidants cause aggregation and/or morphological changes which can be detected by using both image analysis or by colorimetry.

Extraction-free colorimetric determination of thymol and carvacrol isomers in essential oils by pH-dependent formation of gold nanoparticles

An extraction-free method is described for the colorimetric determination of thymol (TY) and carvacrol (CA) isomers in essential oils by making use of the pH-dependent formation of gold nanoparticles (AuNPs). In solutions of pH 12, TY and CA form gold nanoparticles, while at pH ≤ 11 only CA does so. By taking advantage of this finding, two different approaches based on colorimetric assay (absorption at 550 nm) were developed: one at pH 12 for the determination of total CA and TY, and other at pH 9 and pH 12 for differential quantification of TY and CA. The former agrees with the well-established Folin-Ciocalteu method, and the latter provides a simple way for calculation of TY/CA ratio. The linear ranges are from 100 to 1000 μM at pH 9, and from 50 to 200 μM at pH 12. The limits of detection are 0.09 μM at pH 9, and 0.02 μM at pH 12. These features make this method simple, fast and reliable. Conceivably, it can be used to assess the quality of essential oils and may become a valuable alternative to more sophisticated, laborious and high time-consuming methods. Graphical abstract Schematic of the assay: At pH 12 (blue color), thymol and carvacrol form gold nanoparticles (Au), while at pH 9 (red color) only carvacrol does so. This finding resulted in a colorimetric method for the differential quantification of both compounds in essential oils.

Simple and rapid silver nanoparticles based antioxidant capacity assays: Reactivity study for phenolic compounds

A single-step, rapid (10 min), sensitive silver nanoparticles (AgNPs) based spectrophotometric method for antioxidant capacity (AOC) assay has been developed. The assay is based on the ability of natural polyphenols to reduce Ag(I) and stabilize the produced AgNPs(0) at room temperature. Localized surface plasmon resonance (LSPR) of AgNPs at ≈420 nm is then measured. Using different conditions of pH (8.4) and temperature (45 °C) a further assay based on the production of AgNPs with selectivity for flavonols was also developed. The reactivity of the two AgNPs based assays vs. 15 polyphenols belonging to different chemical classes and 9 different samples has been studied and compared with ABTS, Folin and AuNPs based methods for AOC. The proposed assays had good reproducibility (RSD ≤ 13) and are simple, sensitive and cost effective. Moreover, used in conjunction with the classical AOC assays, can improve the information on the polyphenolic pool of food samples.

Nanomaterial-Based Sensing and Biosensing of Phenolic Compounds and Related Antioxidant Capacity in Food

Polyphenolic compounds (PCs) have received exceptional attention at the end of the past millennium and as much at the beginning of the new one. Undoubtedly, these compounds in foodstuffs provide added value for their well-known health benefits, for their technological role and also marketing. Many efforts have been made to provide simple, effective and user friendly analytical methods for the determination and antioxidant capacity (AOC) evaluation of food polyphenols. In a parallel track, over the last twenty years, nanomaterials (NMs) have made their entry in the analytical chemistry domain; NMs have, in fact, opened new paths for the development of analytical methods with the common aim to improve analytical performance and sustainability, becoming new tools in quality assurance of food and beverages. The aim of this review is to provide information on the most recent developments of new NMs-based tools and strategies for total polyphenols (TP) determination and AOC evaluation in food. In this review optical, electrochemical and bioelectrochemical approaches have been reviewed. The use of nanoparticles, quantum dots, carbon nanomaterials and hybrid materials for the detection of polyphenols is the main subject of the works reported. However, particular attention has been paid to the success of the application in real samples, in addition to the NMs. In particular, the discussion has been focused on methods/devices presenting, in the opinion of the authors, clear advancement in the fields, in terms of simplicity, rapidity and usability. This review aims to demonstrate how the NM-based approaches represent valid alternatives to classical methods for polyphenols analysis, and are mature to be integrated for the rapid quality assessment of food quality in lab or directly in the field.

A Colorimetric Sensor for the Highly Selective Detection of Sulfide and 1,4-Dithiothreitol Based on the In Situ Formation of Silver Nanoparticles Using Dopamine

Hydrogen sulfide (H₂S) has attracted attention in biochemical research because it plays an important role in biosystems and has emerged as the third endogenous gaseous signaling compound along with nitric oxide (NO) and carbon monoxide (CO). Since H₂S is a kind of gaseous molecule, conventional approaches for H₂S detection are mostly based on the detection of sulfide (S^2-) for indirectly reflecting H₂S levels. Hence, there is a need for an accurate and reliable assay capable of determining sulfide in physiological systems. We report here a colorimetric, economic, and green method for sulfide anion detection using in situ formation of silver nanoparticles (AgNPs) using dopamine as a reducing and protecting agent. The changes in the AgNPs absorption response depend linearly on the concentration of Na₂S in the range from 2 to 15 μM, with a detection limit of 0.03 μM. Meanwhile, the morphological changes in AgNPs in the presence of S^2- and thiol compounds were characterized by transmission electron microscopy (TEM). The as-synthetized AgNPs demonstrate high selectivity, free from interference, especially by other thiol compounds such as cysteine and glutathione. Furthermore, the colorimetric sensor developed was applied to the analysis of sulfide in fetal bovine serum and spiked serum samples with good recovery.

Visual Monitoring of Food Spoilage Based on Hydrolysis-Induced Silver Metallization of Au Nanorods

Colorimetric detection of biogenic amines, well-known indicators of food spoilage, plays an important role for monitoring of food safety. However, common colorimetric sensors for biogenic amines suffer from low color resolution or complicated design and intricate output for the end-users. Herein, we explored a simple but effective strategy for visual monitoring of biogenic amines with multiple color change based on hydrolysis-induced silver metallization reaction to tune the localized surface plasmon resonance (LSPR) adsorption of Au nanorods (NRs). The color change and blue shift of longitudinal LSPR peak of Au NRs were closely related to the concentration of biogenic amines. This strategy provided a simple, sensitive, robust, nondestructive, cost-effective, and user-friendly platform for in situ evaluating the freshness of foodstuffs.