Gold Nanoparticles-based Extraction-Free Colorimetric Assay in

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Gold Nanoparticles-based Extraction-Free Colorimetric Assay in Organic Media: An Optical Index for Determination of Total Polyphenols in Fat-Rich Samples Flavio Della Pelle,†,‡ María Cristina González,‡ Manuel Sergi,† Michele Del Carlo,† Dario Compagnone,*,† and Alberto Escarpa*,‡ †

Faculty of Bioscience and Technology for Food, Agriculture and Environment, University of Teramo, 64023 Teramo, Italy Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering, Faculty of Biology, Environmental Sciences and Chemistry, University of Alcalá, E-28871 Alcalá de Henares, Madrid, Spain

‡

S Supporting Information *

ABSTRACT: In this work, a rapid and simple gold nanoparticle (AuNPs)-based colorimetric assay meets a new type of synthesis of AuNPs in organic medium requiring no sample extraction. The AuNPs synthesis extraction-free approach strategically involves the use of dimethyl sulfoxide (DMSO) acting as an organic solvent for simultaneous sample analyte solubilization and AuNPs stabilization. Moreover, DMSO works as a cryogenic protector avoiding solidification at the temperatures used to block the synthesis. In addition, the chemical function as AuNPs stabilizers of the sample endogenous fatty acids is also exploited, avoiding the use of common surfactant AuNPs stabilizers, which, in an organic/aqueous medium, rise to the formation of undesirable emulsions. This is controlled by adding a fat analyte free sample (sample blank). The method was exhaustively applied for the determination of total polyphenols in two selected kinds of fat-rich liquid and solid samples with high antioxidant activity and economic impact: olive oil (n = 28) and chocolate (n = 16) samples. Fatty sample absorbance is easily followed by the absorption band of localized surface plasmon resonance (LSPR) at 540 nm and quantitation is refereed to gallic acid equivalents. A rigorous evaluation of the method was performed by comparison with the well and traditionally established Folin−Ciocalteu (FC) method, obtaining an excellent correlation for olive oil samples (R = 0.990, n = 28) and for chocolate samples (R = 0.905, n = 16). Additionally, it was also found that the proposed approach was selective (vs other endogenous sample tocopherols and pigments), fast (15−20 min), cheap and simple (does not require expensive/complex equipment), with a very limited amount of sample (30 μL) needed and a significant lower solvent consumption (250 μL in 500 μL total reaction volume) compared to classical methods.

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to quantify particular classes of polyphenols.7−9 Further elucidation of the chemical structure and quantification of some minor compounds can be obtained using LC-MS methods10,11 but not on a routinely basis. In all the approaches stated before, the extraction step is always necessary to remove the possible interfering compounds, separate the polyphenols from the lipids, and, possibly, to concentrate the polyphenols. The use of nanotechnology has opened a wide range of horizons for its applications in a wide range of fields. Nanomaterials are materials with sizes or features ranging from 1 to 100 nm in one or more dimensions that exhibit some remarkable specific properties significantly different from the properties of bulk material due to their large fraction of surface atoms, large surface energy and spatial confinement and reduced imperfections. Nanomaterials have special thermal,

olyphenols are a class of chemical compounds of considerable interest in the food industry for their nutritional, antimicrobial and sensory proprieties.1−3 Recent studies have shown that this heterogeneous class of substances possesses a protective effect on human health when consumed in significant levels because of their antioxidant properties.4,5 For this reason, the determination of polyphenolic compounds continues to attract considerable research efforts. The classical methods for the determination of phenolic compounds6 require, in all the cases, an extraction step. The extraction techniques mainly used for this purpose are solid phase extraction (SPE) and liquid−liquid extraction (LLE). Both methodologies use large amounts of solvents and require different steps resulting in long times of analysis. Furthermore, the extraction step can affect the accuracy of the procedure due to uncomplete recoveries. Extraction is usually followed by Folin−Ciocalteu (FC) spectrophotometric assay or HPLCUV−vis. Different electrochemical approaches have been also used for the quantification of the total polyphenolic content or © XXXX American Chemical Society

Received: April 21, 2015 Accepted: May 29, 2015

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analyzed. The samples were kept at a temperature of +4 °C in the dark. Apparatus. Absorbance measurements were performed using a Lambda 35 diode array spectrophotometer from PerkinElmer. The transmission electron microscopy (TEM) measurements were carried out using a Zeiss EM10C transmission electron microscope. Determination of Total Polyphenols Using Colorimetric Gold Nanoparticles Assay. 30 μL of olive oil was mixed with 210 μL of DMSO and stirred for 1 min with an orbital shaker (or vortex). 25 μL of HAuCl4 solution (2.0 × 10−2 mol L−1) were then added, and the volume was brought to 500 μL with phosphate buffer solution (pH 8.0; 1.0 × 10−2 mol L−1). The solution was again stirred for 1 min with an orbital shaker and the absorbance at 540 nm taken (time zero). Reaction was started heating for 5 min at 45 °C in a water bath. Finally, the reaction mixture was blocked at −20 °C for 5 min. The absorbance due to the AuNPs formation was recorded at 540 nm (the absorbance at time zero was subtracted). For the chocolate samples, 100 mg of sample was dissolved with the appropriate dilution in DMSO at 45 °C for 10 min, preceded and followed by 1 min of stirring. Samples were then filtered (syringe filter 0.20 μm PTFE). 30 μL of the sample was then reacted with the procedure previously described. Determination of Total Polyphenol by Using the Folin−Ciocalteu Method. The total polyphenol content (TP) of the extracts was evaluated colorimetrically using the Folin−Ciocalteu reagent. The method was adapted from Singleton and Rossi (1965). A LLE system was used to extract the phenolic compounds present in EVOO. According to Pirisi et al. (2000), 2 g of oil was first dissolved in 1 mL of hexane, and then extracted with 4 × 2 mL of methanol/water (60:40, v/ v) solution. The final residue was dissolved in 0.5 mL of methanol/water (50:50, v/v) and stored at −20 °C. The combined extracts of the hydrophilic layer were brought to dryness in a rotary evaporator under reduced pressure and a temperature of 30 °C in the dark. A diluted EVOO extract was mixed with 0.5 mL of Folin−Ciocalteu reagent and 4 mL of deionized water, and the solution was allowed to react for 3 min. 1.5 mL of a 20% solution of Na2CO3 was then added; the volume was finally brought to 10 mL with deionized water. Solutions were stirred at room temperature for 60 min, and the total polyphenols were determined colorimetrically at 725 nm. Calibration curves were realized using gallic acid standard solutions.

mechanical, optical, electrical, magnetic and biological properties, which are size dependent being able to be tuned simply by adjusting the size, shape and extent of agglomeration. In this context, gold nanoparticles (AuNPs) have been employed in a vast set of applications in analytical chemistry.12 As it has recently been reviewed,13 one important piece of work has been dedicated to the use of AuNPs for the evaluation of antioxidant activity. In fact, it has been reported that the endogenous polyphenols present in food samples are able to drive the synthesis of AuNPs, reducing gold(III) to gold(0).14−16 In this current work, a rapid and simple AuNPs-based colorimetric assay meets a new type of synthesis of AuNPs in organic medium requiring no sample extraction. This valuable feature is exploited to develop an extraction-free assay for total polyphenols assessment in fatty matrices with high food significance: olive oils and chocolates samples. Indeed, despite that the major source of polyphenols consumed in diets comes from fruits and vegetables and beverages such as tea and wine, two important fat-enriched samples rich in these compounds are consumed: olive oil and chocolate. Indeed, virgin olive oil is a fundamental ingredient of the Mediterranean diet, and is recognized as a valuable source of natural phenolic antioxidants.4,17,18 Among vegetable oils, extra virgin olive oil (EVOO) exhibits the highest resistance to oxidation, due to its fatty acid composition, characterized by a high monounsaturated- to-polyunsaturated ratio and the presence of a large amount of compounds with antioxidant activity, the majority being polyphenols. On the other hand, (dark) chocolates and chocolate pastes are also particularly rich in polyphenols and are a source of polyphenols quantitatively important for a diet. Chocolate products are rich in flavan-3-ol and the corresponding polymers; these compounds play an important role as antioxidants, seem to have positive effects on human cardiovascular diseases showing anticarcinogen and neuro-preventive properties.19

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MATERIALS AND METHODS Reagents, Stock Solutions, and Reference Compounds. Tyrosol, hydroxytyrosol and oleuropein were obtained from Extrasynthèse (Genay, France). Gallic acid, 3,4-dihydroxyphenilacetic acid, caffeic acid, clorogenic acid, epicatechin, quercetin, gold chloride trihydrate (HAuCl4·3H20 99.9%), Folin−Ciocalteu reagent, sodium carbonate, phenolphthalein, potassium hydroxide, starch, potassium iodide, sodium thiosulfate, disodium hydrogen phosphate anhydrous, sodium dihydrogen phosphate hydrate, dimethyl sulfoxide (DMSO), (+)-α-tocopherol (extracted from olive oil; standard type V, ∼1000 IU/G), olive oil (highly refined, low acidity), hexane, isopropyl alcohol, chloroform, acetic acid, formic acid, acetonitrile, diethyl ether, ethanol and methanol were bought from Sigma Chemicals Co. (St. Louis, MO). All stock solutions of polyphenol standards (in methanol and in DMSO) were prepared at a concentration of 1.0 × 10−2 mol L−1, and stored at −32 °C in the dark. Samples. EVOO sample were obtained from different batches of mixed varieties in an industrial olive mill. Sampling was carried out in the year 2012−2013 and 28 representative samples were selected in order to perform the analyses. The samples were stored at room temperature in darkened glass bottles. Chocolate and cocoa mass samples were obtained from a chocolate industry (Puratos, division Belcolade, Belgium); different production batches of the years 2012−2013 were

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RESULTS AND DISCUSSION Synthesis Route for AuNPs in Organic Media Using Endogenous Polyphenols from Fat Samples: Analytical Strategy for Sample Analysis and Calibration. Scheme 1 illustrates for both sample analysis and calibration the AuNPs synthesis extraction-free approach which involves the use of a dimethyl sulfoxide (DMSO) as organic solvent for sample analyte solubilization and AuNPs stabilization. DMSO works also as a cryogenic protector avoiding solidification at the temperatures used to block the synthesis. In addition, the chemical function as AuNPs stabilizers of the endogenous fatty acids from the fat-real sample (in sample analysis) or fat-blank sample (real sample containing no analyte in standard calibration) is also exploited. This avoids the use of common surfactant AuNPs stabilizers (CTAC, CTAB), which in an organic/aqueous medium, rise to the formation of undesirable B

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aggregates (black/gray color). In addition, the use of stabilizers (CTAC, CTAB), being an organic/aqueous medium, gave rise to the formation of emulsions and, as a consequence, their use is not desirable. The AuNPs suspensions resulted stable in DMSO and endogenous sample fatty acids. In our opinion, fatty acids themselves and their carboxylic groups works in this case as surfactants (particularly at alkaline pH), helping the stabilization/formation of AuNPs. Remarkably, it has been observed that AuNPs are not formed, in the experimental conditions used, in the absence of the sample, demonstrating the suitability of the synthesis route proposed. On the other hand, following the analytical concepts used for sample analysis, a calibration protocol was rigorously studied. To this purpose, the ability of gallic acid (classical standard used for the total polyphenols quantification) as well as the ability of endogenous polyphenols most commonly found in olive oil (tyrosol, hydroxytyrosol and oleuropein) and chocolate (epicatechin, caffeic acid and quercetin) to drive formation of AuNPs using the lipid matrix have been demonstrated. The ability of gallic acid to drive formation of AuNPs using the protocol developed is clearly shown in Figure 1, where spectra from 400 to 800 nm are reported after the addition of gallic acid to refined olive oil (oil sample without polyphenols) (A) and gallic acid added to a mix of chocolate and cocoa mass sample (B)). Refined oil and chocolate samples were properly used to stabilize the AuNPs generated from gallic acid as external standard chosen for quantification of total polyphenols in both kinds of samples. The calibration curve obtained with increasing concentrations of gallic acid from 25 to 225 mg L−1, taking the absorbance at 540 nm exhibited an excellent linearity (y = 0.0062x + 0.0066; R2 = 0.994). The limit of detection (LOD) calculated as 3 times the standard deviation of the blanks (n = 20) was 35 mg L−1. The limit of quantification, calculated as 10 times the standard deviation of the blanks (n = 20) was 43 mg L−1. The spectra of chocolate samples fortified with increasing concentrations of gallic acid, reported in Figure 1B, exhibited the same absorbance peak at 540 nm. Quantitative and

Scheme 1. Analytical Strategy for Sample Analysis and Calibration Using the AuNPs Synthesis Free-Extraction and Free-Surfactant Approach for Determination of Total Polyphenols in Fat-Enriched Samples

emulsions, making this free-surfactant procedure a pertinent and suitable approach for these kinds of samples. The synthesis was carefully optimized. The ratio of reagents/ solvents, reaction times and temperatures (in both progress and stopping reaction) were optimized, being the amount of DMSO is a critical factor. Indeed, for volumes of DMSO higher than 240 ± 20 μL (in a final volume of 500 μL), formation of AuNPs was not noticeable whereas volumes of DMSO lower than 240 ± 20 μL (in a final volume of 500 μL) did not allow optimum solubilization of the phenolic compounds from the matrix. The use of the organic solvent/aqueous ratio allowed the blocking of the reaction at lower temperatures (T < 0 °C), because of the decrease of the freezing point of the mixture (≈−45 °C) with respect to pure DMSO (−18 °C). Furthermore, synthesis times longer than 5 min lead to AuNPs aggregation and/or grow in size shifting the LSPR maximum. 10 and 15 min incubation times resulted in an absorbance maxima around 600 and 650 nm, respectively, with lower intensity of the signal indicating the formation of

Figure 1. (A) AuNPs spectra obtained with increasing concentration of gallic acid in refined olive oil (from 25 to 225 mg L−1). The inset photographs show the AuNPs formed with gallic acid in refined olive oil (125 mg L−1) and the relative blank (left). (B) AuNPs spectra formed with a mix of dark chocolate and cocoa mass samples (CS) and with the sample spiked with three increasing concentrations of gallic acid (a, b, c: 50, 75, 100 mg L−1 of gallic acid, respectively); BS represents the spectrum at time zero of the reaction (black line). The inset photographs show the AuNPs formed after addition of gallic acid (75 mg L−1) in a mix of dark chocolate and cocoa mass samples, and the relative solution at time zero of the reaction (left). C

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Figure 2. (A) Calibration curves obtained for increasing concentrations of EVOO polyphenols standards (absorbance at 540 nm): (×) hydroxytyrosol (y = 1.5371x − 0.0054; R2 = 0.992), (▲) oleuropein (y = 0.7525x + 0.0509; R2 = 0.993) and (■) tyrosol (y = 0.4237x + 0.0375; R2 = 0.995). (B) Calibration curves obtained for increasing concentrations of chocolate polyphenols standards (absorbance at 540 nm): (×) quercetin (y = 29.59x − 0.1756; R2 = 0.993), (■) caffeic acid (y = 27.136x − 0.3841; R2 = 0.993) and (▲) epicatechin (y = 17.414x − 0.0753; R2 = 0.994).

Figure 3. TEM micrographs of AuNPs formed using gallic acid (A), tyrosol (B) and epicatechin (C).

Figure 4. (A) AuNPs spectra corresponding to two selected oil samples (15 and 21, see Table 1), and one typical spectrum at time zero of the reaction (black line). The inset photograph shows the AuNPs formed with the two oil samples, and a typical solution at time zero of the reaction (left). (B) AuNPs spectra corresponding to chocolate and cocoa mass samples (4 and 11, see Table 1), and one typical spectrum at time zero of the reaction (black line). The inset photograph shows the AuNPs formed with the chocolate and cocoa mass samples, and a typical solution a time zero of the reaction (left).

reproducible recoveries (calculated on refined olive oil calibration curve) were obtained with values ranging between 90 and 110% (RSD < 7%, n = 3). Calibration curves were also carried out using the lipid matrix for commonly distributed polyphenols in olive oil (tyrosol, hydroxytyrosol and oleuropein) and chocolate (epicatechin,

caffeic acid and quercetin). The absorbance maximum was in all cases at λ = 540 nm. Figure 2A reports the calibration curves of the AuNPs obtained with increasing concentrations of tyrosol, hydroxytyrosol and oleuropein, three of the major polyphenols found in olive oil. Figure 2B reports calibration curves obtained with the three typical polyphenols of chocolate, caffeic acid, D

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Figure 5. TEM micrograph of AuNPs formed using EVOO (A) and chocolate (B) samples.

Table 1. Analytical Determination of Total Polyphenols with the AuNPs-based Assay (TPAuNPs) and FC (TPFC) Methods (n = 3) olive oil sample

TPAuNPs (mg L−1)

RSD (%)

TPFC (mg L−1)

RSD (%)

chocolate sample

TPAuNPs (mg L−1)

RSD (%)

TPFC (mg L−1)

RSD (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

108 291 100 48 159 120 49 153 304 50 159 209 235 93 53 110 128 156 115 77 90 128 41 304 366 301 320 314

6 9 15 10 4 5 13 6 8 7 5 6 8 1 9 3 5 15 3 6 13 5 9 12 5 15 14 6

94 274 92 61 158 120 57 145 312 57 138 213 229 82 71 111 140 178 141 105 124 143 61 299 362 314 312 348

8 6 12 14 4 4 18 12 6 3 7 4 2 9 16 8 1 2 1 9 11 9 14 11 8 5 8 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1718 3973 1819 2038 2466 2982 2038 4511 931 1598 976 766 1025 567 995 767

11 14 11 5 12 9 1 5 14 12 14 13 7 9 11 10

1465 3437 2236 2361 2058 2315 2602 3887 1135 1151 631 1734 1830 453 735 1227

7 2 8 9 1 3 10 6 4 4 2 5 4 3 6 8

epicatechin, and quercetin, each belonging to different chemical classes (hydroxicinammic acids, flavan-3-ols and flavonols, respectively). All the calibration curves had a good value of determination coefficient (R2 ≥0.992). It should be noticed that these calibrations were performed to demonstrate that the prominent sample polyphenols are driving the AuNPs synthesis and that absorbance was linearly dependent on the polyphenol concentration. They were not used for sample polyphenol quantitation since all polyphenols were quantified as gallic acid equivalents. Figure 3 shows selected TEM images corresponding to the AuNPs generated from the gallic acid (external standard) (A), tyrosol as representative of oil samples (B) and epicatechin as representative of chocolate samples. (C). According to the redstable color of gold solutions, in all cases, a well-defined spherical AuNPs with a ϕ < 20 nm was observed. These results

demonstrate that these polyphenols are generating qualitatively and quantitatively AuNPs. Sample Characterization and Analysis: Analytical Evaluation of the Method. Absorption spectra and colloidal solutions (inset) of two-selected oil (A) and two-selected chocolate (B) samples (in triplicate) are shown in Figure 4. The absorbance maximum was in all cases at λ = 540 nm and colloidal solutions exhibited a strong red color indicative of the AuNPs. No formation of AuNPs was noticed in the absence of samples showing noncolored solutions. The presence of AuNPs in the fat-samples was further confirmed by TEM. Indeed, Figure 5 shows selected TEM images corresponding to the AuNPs generated from the oil olive (A), and chocolate samples (B). In both samples, a welldefined spherical AuNPs was also observed (ϕ < 20 nm) accordingly to the red color observed. These results confirmed E

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solvent−aqueous medium, driven by endogenous polyphenols present in the food matrix examined. The proposed method does not require sample extraction and it is simply followed by reading absorbance at 540 nm. Olive oil samples with different free acidity, peroxide number and variable content of polyphenols and tochopherols have been assayed together with chocolate samples having different fat content. A very good correlation with the most used method for the determination of total phenolic content, the Folin−Ciocalteu, was obtained for the entire set of oil (r = 0.990, n = 28) and chocolate (r = 0.905 n = 16) samples used. The results obtained demonstrated the suitability of this approach as a valuable tool for the rapid determination of phenolic content in fat rich samples. Furthermore, the proposed procedure compared to the classical FC method requires a very low amount of sample (max 30 μL vs 2 g of oils), involves a significant lower solvent consumption (no extraction, 500 μL total reaction volume vs 10 mL repeated extractions in hexane) and is faster (15−20 min vs at least 60 min in incubation time plus extraction and preconcentration); no expensive/complex equipment is required.

these endogenous polyphenols are generating AuNPs in these organic media. Selectivity of the assay was also carefully tested vs a lipophilic antioxidant as tocopherol that is usually found in olive oil. Addition of different amounts of tocopherol to refined olive oil did not produce measurable AuNPs in the assay, indicating no chemical interference (see Figure S-1a of the Supporting Information). Another important control was also performed. The spectra of the oils at the working dilution factor (6% olive oils) and even for the pure oil without dilutions (although we did not work under these conditions) were also recorded (see Figure S-1b of the Supporting Information). No spectra interference was noticed. Finally, to demonstrate the suitability of this approach in quantitative domains, sample analysis was carried out. Table 1 reports total polyphenolic content using the AuNPs-based assay (TPAuNPs, expressed as gallic acid equivalents) for a wide set of fat-rich samples (28 olive and 16 chocolate samples assayed in triplicate). To perform the analytical evaluation of the AuNPsbased assay, total polyphenols using Folin−Ciocalteu (TPFC) was also carried out. A high agreement with excellent correlations (see Figure S-2 of the Supporting Information) between TPAuNPs and TPFC were obtained for both (olive oil r = 0.990, n = 28) and chocolate (r = 0.905, n = 16) samples. The high correlation between TPAuNPs and TPFC can be attributed to the exploitation of the reducing power of the polyphenols by both methods. Also, for both, TPAuNPs and TPFC methods used for the quantification of polyphenols, good repeatability was obtained. The confirmation and the quantitative determination of the major polyphenols in olive oil and chocolate samples were also carried out using HPLC (see Table S-1 of the Supporting Information). In olive oil samples, hydroxytyrosol, tyrosol and oleuropein were found to vary in the 0−50, 0−37 and 0−256 mg L−1 range, respectively. In chocolate samples, caffeic acid, cathechin and epicathechin were found to vary in the 55−3056, 0−108 and 85−1202 mg L−1 range, respectively. Both the level of oxidation and the amount of free fatty acids was also highly variable in olive oil samples. In fact, the peroxide number was in the 5−46 mequiv O2 kg1− range and free acidity was in the 0.32−1.99 range (as % of oleic acid; see Table S-2 of the Supporting Information). Total fat content of chocolate samples ranged from 24 to 55% (see Table S-3 of the Supporting Information). Considering the good correlation obtained with the FC method, it thus indicates that the method is insensitive to variation of oxidation status, fat and free fatty acid content of the samples. This latter evidence is particularly significant with respect to our statement that carboxylic groups of fatty acids stabilizes AuNPs formation. We can conclude that, in the optimized conditions, the amount of fatty acids present in the blank sample (0.22 as % of oleic acid) is enough for the stabilization of the formed AuNPs and that the additional amount introduced in the reaction medium with samples does not affect the assay. Thus, in light of the high complexity of the samples studied the results obtained could be considered excellent to propose this as a novel AuNPs-based optical index for fat-rich samples.

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ASSOCIATED CONTENT

S Supporting Information *

HPLC-UV/vis determination of the major polyphenols found in olive oil and chocolate samples; thochoperols content, free acidity and peroxide number (mean values n = 3) of 28 olive oil samples; total fat ( of 8 cocoa mass (1−8) and 8 dark chocolate samples (9−16); absorption spectra of refined olive oil and unreacted reaction mixture; correlation of the data obtained with the proposed method with the method of Folin−Ciocalteu for oil (a) and chocolate samples (b). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b01489.

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AUTHOR INFORMATION

Corresponding Authors

*D. Compagnone. E-mail: [email protected]. *A. Escarpa. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial contribution of “Fondazione Tercas”, under the “Progetti di Eccellenza” scheme is acknowledged. Financial support provided by the Spanish Ministry of Science and Innovation (CTQ2011-28135), the NANOAVANSENS program from the Community of Madrid (S2013/MIT-3029) is also gratefully acknowledged. The authors give special thanks to Ana Zdravkovic for helping with the design of the graphical abstract.

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REFERENCES

(1) Bendini, A.; Cerretani, L.; Carrasco-Pancorbo, A.; GómezCaravaca, A. M.; Segura-Carretero, A.; Fernández-Gutiérrez, A.; Lercker, G. Molecules 2007, 12, 1679−1719. (2) Medina, E.; De Castro, A.; Romero, C.; Brenes, M. J. Agric. Food Chem. 2006, 54, 4954−4961. (3) Escarpa, A.; González, M. C. CRC Anal. Chem. 2001, 31, 57−139. (4) Carrasco-Pancorbo, A.; Cerretani, L.; Bendini, A.; SeguraCarretero, A.; Del Carlo, M.; Gallina-Toschi, T.; Lercker, G.; Compagnone, D.; Fernandez-Gutierrez, A. J. Agric. Food Chem. 2005, 53, 8918−8925.

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CONCLUSIONS In this work, a simple AuNPs-based colorimetric assay for the determination of the total polyphenolic content of olive oil and chocolate samples has successfully been developed. The method relies on the formation of AuNPs in organic F

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Analytical Chemistry (5) Williamson, G.; Manach, C. Am. J. Clin. Nutr. 2005, 81, 2435− 2555. (6) Ignat, I.; Volf, I.; Popa, V. I. Food Chem. 2011, 126, 1821−1835. (7) Blasco, A. J.; Rogerio, M. C.; González, M. C.; Escarpa, A. Anal. Chim. Acta 2005, 539, 237−244. (8) Kovachev, N.; Canals, A.; Escarpa, A. Anal. Chem. 2010, 82, 292. (9) Del Carlo, M.; Amine, A.; Haddam, M.; Della Pelle, F.; Fusella, G. C.; Compagnone, D. Electroanalysis 2012, 24, 889−896. (10) Gilbert-López, B.; Valencia-Reyes, Z. L.; Yufra-Picardo, V. M.; García-Reyes, J. F.; Ramos-Martos, N.; Molina-Díaz, A. Food Anal. Methods 2014, 7, 1824−1833. (11) Engström, M. T.; Pälijärvi, M.; Fryganas, C.; Grabber, J. H.; Mueller-Harvey, I.; Salminen, J. P. J. Agric. Food Chem. 2014, 62, 3390−3399. (12) Zamborini, F. P.; Bao, L.; Dasari, R. Anal. Chem. 2012, 84, 541− 576. (13) Vilela, D.; González, M. C.; Escarpa, A. Trends Anal. Chem. 2015, 64, 1−16. (14) Scampicchio, M.; Wang, J.; Blasco, A. J.; Arribas, A. S.; Mannino, S.; Escarpa, A. Anal. Chem. 2006, 78, 2060−2063. (15) Vilela, D.; González, M. C.; Escarpa, A. Anal. Bioanal. Chem. 2012, 404, 341−349. (16) Della Pelle, F.; Vilela, D.; González, M. C.; Lo Sterzo, C.; Compagnone, D.; Del Carlo, M.; Escarpa, A. Food Chem. 2015, 178, 70−75. (17) Manach, C.; Scalbert, A.; Morand, C.; Rémésy, C.; Jiménez, L. Am. J. Clin. Nutr. 2004, 79, 727−47. (18) Briante, R.; Febbraio, F.; Nucci, R. J. Agric. Food Chem. 2003, 51, 6975−6981. (19) Scalbert, A.; Manach, C.; Morand, C.; Remesy, C.; Jimenez, L. Crit. Rev. Food Sci. Nutr. 2005, 45, 287−306.

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