Simple and rapid method for screening of pyrophosphate using 6,6-ionene-stabilized gold and silver nanoparticles

Sensors and Actuators B 241 (2017) 390–397

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Simple and rapid method for screening of pyrophosphate using 6,6-ionene-stabilized gold and silver nanoparticles Ekaterina A. Terenteva a , Viktoria V. Arkhipova a , Vladimir V. Apyari a,∗ , Pavel A. Volkov b , Stanislava G. Dmitrienko a a b

Lomonosov Moscow State University, Chemistry Department Leninskie gory 1/3, 119991 Moscow, Russia Scientific-Research Institute of Chemical Reagents and Special Purity Chemicals, Bogorodsky Val 3, 107076, Moscow, Russia

a r t i c l e

i n f o

Article history: Received 3 March 2016 Received in revised form 18 October 2016 Accepted 20 October 2016 Available online 20 October 2016 Keywords: Colorimetric probe Ionene Gold nanoparticles Silver nanoparticles Aggregation Pyrophosphate Spectrophotometry Naked eye

a b s t r a c t Desensitized ionene-stabilized gold and silver nanoparticles were prepared and applied as colorimetric probes for single-step determination of pyrophosphate at the relatively high concentration level. The approach is based on aggregation of the nanoparticles leading to the change in their absorption spectra and color of the solution. Due to both electrostatic and steric stabilization these nanoparticles show decreased sensitivity, which allows for simple and rapid direct single-step determination of pyrophosphate at the relatively high concentration level in real samples. Influence of different factors (the time of interaction, pH, the concentrations of pyrophosphate and the nanoparticles) on aggregation and analytical performance of the procedure was investigated. The method allows for the determination of pyrophosphate in the mass range of 90–150 ␮g and 45–150 ␮g with RSD of 2–5% by using gold and silver NPs, respectively. It has sharp dependence of the colorimetric response on the concentration of pyrophosphate, which makes it prospective for indicating deviations of the concentration regarding some declared value chosen within the above mentioned ranges using spectrophotometry or naked-eye detection. The method was applied to the analysis of baking powder sample, copper plating solution and samples of bread. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Pyrophosphates are actively used in industry as food additives to increase the mass of muscle tissue and the yield of a final product. Also pyrophosphates improve organoleptic characteristics and product consistency, they stabilize the color and slow down the oxidative processes. This additive is widely used in canning of various meat products, seafoods and in the preparation of processed cheese. It may also be contained in some dairy products [1,2]. When excessive use of this additive, it can cause indigestion, as well as disorders related to an imbalance of phosphorus and calcium in the body. With increased use of this additive, deterioration of the calcium absorption may take place, resulting in deposition of phosphorus and calcium in kidneys, which contributes to the development of osteoporosis [3]. The traditional method for determination of pyrophosphate is titrimetry [4]. Electrochemical [1,5], chromatographic [6,7], biological [8,9] and spectrophotometric methods [10–12] have also

∗ Corresponding author. E-mail address: [email protected] (V.V. Apyari). http://dx.doi.org/10.1016/j.snb.2016.10.093 0925-4005/© 2016 Elsevier B.V. All rights reserved.

been proposed for its determination. However, simple and fast spectrophotometric or naked-eye colorimetric procedures are of interest. One of the prospect ways to develop such a procedure can be based on the optical properties of gold and silver nanoparticles (AuNPs and AgNPs). These nanoparticles have attracted the attention in the last two decades due to their unique properties [13–16]. The surface plasmon resonance (SPR) is one of the interesting characteristics of these nanoparticles, which gives them a peculiar optical behavior. As a result, AuNPs and AgNPs display intense colors and corresponding specific extinction bands in their UV–vis spectra (515–530 nm and 390–440 nm, respectively), which depend on the size, shape, dielectric environment and aggregation state of the nanoparticles. The aggregative stability and selectivity of NPs aggregation depends essentially on the type of a stabilizer and functionalizing additives used during their preparation. Hence, functionalizators/organic ligands play important role in tuning NPs applications [17–20]. Currently nanoparticles are widely used in analytical chemistry for spectrophotometric determination of metal cations [21–24], anions [25–28] and organic compounds [29–34]. However, the studies on the determination of anions are much rare than the others.

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Fig. 1. Absorption spectra (a), TEM images and ED patterns (b, c) of the ionene-stabilized gold (b) and silver (c) NPs.

In this study, we investigated the possibility of using gold and silver nanoparticles stabilized with 6,6-ionene for spectrophotometric determination of pyrophosphate based on their aggregation. 2. Materials and methods 2.1. Materials Hydrogen tetrachloroaurate, silver nitrate, sodium borohydride, N,N,N,N,-tetramethylhexamethylenediamine, 1,6-dibromohexane,

391

(ɑ) cNPs = 24 ␮g mL−1 .

N,N-dimethylformamide, acetone, hydrochloric, nitric and phosphoric acid, sodium hydroxide, sodium sulfate, bromide, chloride, perchlorate, chlorate, fluoride, nitrate, phosphate and bicarbonate were used. All substances were at least of analytical grade. The substances stock solutions were prepared by dissolving their weighed portions in deionized water. 6,6-Ionene (poly(N,N-dimethyl hexamethyleneiminium bromide), [−N(CH3 )2 + Br− −(CH2 )6 −]n ) was synthesized according to the following procedure [35]. Equimolar solutions of N,N,N,N,-tetramethylhexamethylenediamine and 1,6-dibromohexane were mixed in N,N-

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Aagg/ASPR 1,4 1,2 1 0,8

Ag NPs Au NPs

0,6 0,4 0,2 0 flu

id or

e

rid lo h c

e om br

e id

te ra lo h c

e te at ra on lo h b r rc ca pe ro d hy

tra ni

te

Fig. 2. Influence of different anions on the aggregation ratio of ionene-stabilized gold and silver NPs.

dimethylformamide up to the final concentration of each substance of 1 mol L−1 . The mixture was stirred using a magnetic stirrer at room temperature until sedimentation of the polymer was completed. The end point of the reaction was controlled visually by evaluation of the sediment volume. The mixture was poured into 20-fold volume of water-free acetone at stirring. The sediment was filtered under vacuum and washed doubly with 10 mL of acetone. The product was dried under vacuum of 10−2 mmHg.

2.2. Instrumentation Absorption spectra of the solutions were recorded by SF103 spectrophotometer (Akvilon, Russia), pH was measured by Ekspert 001 ion meter (Ekoniks, Russia), transmission electron microscopy (TEM) images of the samples and electron diffraction (ED) were recorded using LEO912 AB OMEGA microscope (Carl Zeiss, Germany). Optical emission spectra were recorded using an ICP MS on an iCAP 6300 Duo inductively coupled plasma-atomic emission spectrometer. Also, a magnetic stirrer and a mechanical shaker were used.

2.3. Synthesis of ionene-stabilized gold nanoparticles Gold nanoparticles stabilized with 6,6-ionene were prepared by reducing metal salt precursor (hydrogen tetrachloroaurate, HAuCl4 ) by sodium borohydride in the presence of 6,6-ionene. The method of synthesis was developed in our laboratory previously [27,36]. Briefly, 0.05 g portion of 6,6-ionene was placed in a round bottom bulb, dissolved in 18.5 mL of deionized water; and 6.5 mL of 0.1 mol L−1 HCl was injected at stirring. Then 25 mL of a solution containing 1.25 mL of 1% HAuCl4 were introduced in the bulb dropwise. The brown solution was stirred for 15 min. Then a solution of 0.025 g NaBH4 in 50 mL of water was added dropwise into the bulb at vigorous stirring. The color of the solution was changed to ruby. The solution was stirred for 30 min and kept for 24 h to achieve re-crystallization and complete stabilization of AuNPs. The concentration of AuNPs in the final solution was 70 ␮g mL−1 (0.35 mmol L−1 in terms of gold).

e e at at ph ph s s o o ph ph ro ro y d p hy di

te lfa su

canion = 0.25 mg mL−1 , cNPs = 24 ␮g mL−1 Au, cNPs = 6 ␮g mL−1 Ag.

2.4. Synthesis of ionene-stabilized silver nanoparticles Silver nanoparticles stabilized with 6,6-ionene were also prepared by reducing metal salt precursor (silver nitrate, AgNO3 ) by sodium borohydride in the presence of 6,6-ionene. Briefly, 0.05 g portion of 6,6-ionene was placed in a round bottom bulb and dissolved in 25 mL of deionized water at stirring. Then 25 mL of a solution containing 3.75 mL of 10−2 mol L−1 AgNO3 were added in the bulb dropwise. The solution was stirred for 15 min. Then a solution of 7 mg of NaBH4 in 50 mL of water was added dropwise into the bulb at vigorous stirring. The color of the solution was changed to brown. The solution was stirred for 30 min and kept for 24 h to achieve re-crystallization and complete stabilization of AgNPs. The concentration of AgNPs in the final solution was 38 ␮g·mL−1 (0.35 mmol L−1 in terms of silver). The concentration of AuNPs and AgNPs was determined using previously constructed calibration curves. Efficiency of conversion of ionic forms to metals was almost 100% as a result of extremely high reductive ability of NaBH4 . Gold and silver NPs prepared by these techniques were characterized by UV–vis absorption spectroscopy, TEM and ED (Fig. 1). The synthesized ionene-stabilized AuNPs had the average diameter of 16 nm and a surface plasmon resonance (SPR) band at 520 nm in water solution; AgNPs had the average diameter of 23 nm and a surface plasmon resonance (SPR) band at 400 nm. 2.5. Calibration curves plotting Various amounts of pyrophosphate up to 0.41 mg as aliquots of a working solution were introduced into a test-tubes and diluted with deionized water up to 3 mL. Then 1 mL of NPs solution (c = 24 ␮g mL−1 Au, c = 13 ␮g mL−1 Ag) was added. The absorption spectra were recorded after 2 min in case of AuNPs and 10 min in case of AgNPs. The calibration curves were plotted as the Aagg /ASPR ratio, which was A675 /A520 for AuNPs and A500 /A400 for AgNPs, versus the concentration of pyrophosphate. 2.6. Pretreatment of the solution of baking powder The sample of baking powder was dissolved in deionized water. Hydrochloric acid was added under heating for removal of carbon

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393

Fig. 3. Absorption spectra of Au NPs (a) and Ag NPs (b) solutions at different pyrophosphate concentrations, TEM images and ED patterns (c, d) of Au NPs (c) and Ag NPs (d) after interaction with pyrophosphate. (a): cNPs = 12 ␮g mL−1 Au, t = 2 min; cpuroph , ␮g mL−1 : 0, 10, 13, 15, 17, 20, 25, 30, 35, 50. (b): cNPs = 12 ␮g mL−1 Ag, t = 10 min; cpuroph , ␮g mL−1 : 0, 2, 5, 8, 12, 16, 20, 30, 50, 70.

dioxide. Then the solution was adjusted to the desired pH value by sodium hydroxide. The solution was made up to the mark with deionized water, then it was diluted. An aliquot portion of the solution was taken for the analysis.

2.7. Pretreatment of bread samples Two weighed (1.00 g) samples of bread (loaf of “Bakery №22” company, Moscow, Russia, and wheat sandwich bread “American Sandwich” of “Harris CIS” Company, Solnechnogorsk, Russia) were dissolved in deionized water. Additives of pyrophosphate were made to the obtained solutions. The samples were mixed with a mechanical shaker for 20 min. After that, aliquots of solution were passed through a paper filter and used for further analysis.

3. Results and discussion In our previous work [27], we demonstrated that the desensitized 6,6-ionene-stabilized AuNPs could be successfully applied for determination of sulfate. These NPs are positively charged to allow effective interaction with multi-charged anions and quite stable to exclude their aggregation at the low concentrations of the anions and to diminish interferences from single-charged species. Being a cationic polymer, 6,6-ionene is adsorbed on the metal surface, stabilizing NPs owing to the electrostatic repulsion between positively charged ionene polymeric chains as well as owing to the steric hindrance to NPs aggregation produced by the hexamethylene fragments. The ionene-modified AuNPs are stable in solution at least for 4 month. Their destabilization can be achieved in the presence of substances that can effectively decrease the surface electrostatic potential of NPs or substances able to interact with

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two or more NPs, leading to their cross-linking. In the present study, we synthesized two types of 6,6-ionene-stabilized NPs − AuNPs and AgNPs, and investigated prospects of their use for detection of pyrophosphate at the relatively high concentration level. When adding different anions up to the concentration of 0.25 mg mL−1 , it was found that only pyrophosphate and sulfate caused aggregation of NPs. Dihydrophosphate, nitrate, chlorate, perchlorate, bromide, fluoride, chloride and hydrocarbonate did not affect the NP aggregative state remarkably. The aggregation became apparent with the change of color of the solution from ruby to blue for AuNPs and from yellow to gray-pink for AgNPs, and could be characterized by the absorbance ratio at 675 and 520 nm (A675 /A520 ) and at 500 and 400 nm (A500 /A400 ), respectively (Fig. 2). The electron spectrum of the NP solution had a new band at 700 or 500 nm, which corresponded to the gold or silver NP aggregates, respectively. The SPR band of individual NPs at 520 nm or 400 nm was decreased. TEM-image of the sample showed the presence of differently shaped bunch-like aggregates whereas ED pattern had no change in the stimuli positions that proved immutability of a metal core during the interaction (Fig. 3). We assume that the effect observed is connected with different charge and size of these anions, and hence, the different ability to form bonds with positively charged NPs [37]. The sorption of pyrophosphate on the NP surface results in decrease in positive charge and repulsion of NPs as well as gives rise to their crosslinking via negatively charged pyrophosphate and positively charged ionene. Both these factors promote aggregation.

Presumed scheme of Au NP aggregation under the influence of pyrophosphate ions. 3.1. Influence of different factors on aggregation of NPs in the presence of pyrophosphate Influence of the time of interaction, pH and the concentrations of NPs and pyrophosphate on the aggregation was investigated. The degree of aggregation was estimated, comparing the absorption spectra and ratios of absorbances Aagg /ASPR at wavelengths of the aggregates and individual NPs absorption. According to the literature [32,38], this ratio is often used for calibration curve plotting when determining metals and organic compounds. 3.1.1. Influence of the time of interaction Duration of interaction between NPs and pyrophosphate strongly effects on the Aagg /ASPR (A675 /A520 for Au NPs and A500 /A400 for Ag NPs) aggregative ratio. The dependence of this ratio on the time of interaction is given in Fig. 4. One can see that the interaction proceeds in less than 2 min in case of AuNPs and 10 min in case of AgNPs. This time interval allows getting the maximum yield of gold and silver NP aggregates. 3.1.2. Influence of pH The dependence of NP aggregation on pH was investigated in absence and in the presence of pyrophosphate. The acidity required was adjusted with H3 PO4 or HNO3 and NaOH solutions. Corresponding dependences of the change in the Aagg /ASPR aggregative ratio upon addition of pyrophosphate are depicted in Fig. 5. Accord-

ing to the figure, pyrophosphate causes aggregation at pH 5–8.5 for AuNPs and at pH 7–9 for AgNPs. The decrease in the effect of pyrophosphate at pH <5 seems to be connected with its protonation (pKa(HL) = 9.4; pKa(H2 L) = 6.8; pKa (H3 L) = 1.7; pK (H4 L) = 1.0) leading to decrease in the charge. At pH >11 for Au NPs and at pH > 9 for Ag NPs, aggregation of NPs even in absence of pyrophosphate can be observed, which is probably connected with their destabilization by OH− ions. pH was not adjusted in all further investigations, i.e. it was 8.5. 3.1.3. Influence of pyrophosphate and NP concentration Fig. 6 demonstrates the dependences of the Aagg /ASPR ratio on the concentration of pyrophosphate at the different concentrations of NPs. Pyrophosphate causes aggregation of AuNPs at the concentrations of >30 ␮g mL−1 and aggregation of AgNPs at the concentrations of >10 ␮g mL−1 . At higher concentrations of pyrophosphate, a drastic increase in the aggregative ratio can be observed, and after that each dependence reaches saturation. The dependences are quite sharp, which can be used for sensitive monitoring deviations of pyrophosphate concentration relatively to a certain declared value from this range. It could be useful for screening samples for correspondence to this value. For standard samples of different type, with different contents of pyrophosphate, the final concentration of this ion can be adjusted to this value by the choice of a sample aliquot. One can see that shape of these curves is affected by the concentration of NPs in solution. Increasing concentration of NPs leads to

a shift in the range of the pyrophosphate concentrations inducing aggregation. In other words, the lower concentration of NPs, the more sensitive to pyrophosphate they are. This can be a tool for changing the sensitivity. 3.2. Analytical features of the method Dependence of the Aagg /ASPR ratio on the concentration of pyrophosphate enables to develop an analytical procedure for its spectrophotometric determination using 6,6-ionene-stabilized gold and silver NPs. The analytical features of the method are as follows: the limit of detection is 50 ␮g mL−1 , the analytical range is 60–80 ␮g mL−1 in case of AuNPs, and the limit of detection is 12 ␮g mL−1 , the analytical range is 15–50 ␮g mL−1 in case of AgNPs. The limits of detection were estimated as the concentration of pyrophosphate corresponding to the change of Aagg /ASPR ratio equal to 3 times standard deviation of the base signal. The possible reason for lower LODs achieved with AgNPs consists in their lower stabilization compared to AuNPs. It becomes clear taking into account solubility constants of AgBr (Ks = 5.3 10−13 ) and AuBr (Ks = 5.3 10−17 ). The lower value for AuBr indicates stronger adsorption of Br− by the Au surface, hence, higher stabilization of AuNPs, which results in their higher aggregative stability and higher LODs. The analytical features for NPs of different type and concentration are represented in Table 1. One can see that decreasing concentration of NPs leads to lower limits of detection; however the analytical range becomes narrower. At the

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А agg/A SPR 1,4

395

ΔAagg/ASPR 1

11 1

1,2

0,8

0,8

0,6

0,6

0,6

0,4

0,4 2

0,4

2 2

0,2

2

0,2

0,2

0

0 0

2

4

6

8

10

0

12 t, min

2

0

4

6

8

10

aggregation of AuNPs

0,8

aggregation of AgNPs

1

12

рН

Fig. 4. Dependence of the aggregative ratio (Aagg /ASPR ) of AuNPs (1) and AgNPs (2) in the presence of 70 ␮g mL−1 and 17 ␮g mL−1 of pyrophosphate on the time of interaction. cNPs = 6 ␮g mL−1 ; cNPs = 24 ␮g mL−1 .

Fig. 5. Dependence of the change in the Aagg /ASPR aggregative ratio after addition of pyrophosphate on pH. (1): cAuNPs = 24 ␮g mL−1 , t = 2 min, cpyroph = 70 ␮g mL−1 ; (2): cAgNPs = 6 ␮g mL−1 , t = 10 min, cpyroph = 17 ␮g mL−1 .

equal concentration, AgNPs show 2–3 times higher sensitivity than AuNPs. The selectivity of the method was evaluated. It was found that the determination of 70 ␮g mL−1 pyrophosphate was not affected by at least equal amounts of HCO3 − , CH3 COO− , Ca2+ ,Cu2+ , Fe3+ , ATP, 2-times fold of Cl− , 10-times fold of Na+ + K+ , citric acid, and 15-times fold of NO3 − in case of AuNPs; and the determination of 17 ␮g mL−1 pyrophosphate was not affected by at least equal amounts of Cl− , Cu2+ and starch, 5-times fold of CH3 COO− , NH4 + ,10-

times fold of HCO3 − , 15-times fold of Na+ + K+ and 20-times fold of NO3 − in case of AgNPs. It should be mentioned that an important feature of the developed method is easiness in carrying out a semi-quantitative test-determination. The contrast NP color change can be observed upon addition of pyrophosphate, which can be easily distinguished by a naked eye.

a

b А 500 /А 400

А 675 /А 520

1

1,6

1

3

1,4

2

0,8

3

2

1,2 1

0,6

1 0,8

0,4

0,6 0,4

0,2

0,2 0

0 0

20

40

60

80

100

120 140 с pyroph, μg mL-1

0

20

40

60

80

Fig. 6. Dependence of the Aagg /ASPR ratio on the concentration of pyrophosphate at the different concentrations of AuNPs (a) and AgNPs (b). cAuNPs , ␮g mL−1 : 12 (1), 24 (2) and 47 (3); cAgNPs , ␮g mL−1 : 6 (1), 12 (2) and 24 (3).

100 120 с pyroph, μg mL-1

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Table 1 Some ɑnalytical features of the method proposed (n = 3, P = 0.95). cNPs, ␮g mL−1

Analytical range, ␮g mL−1

12 24 47

30–50 50–70 80–100

6 12 24

11–30 15–50 18–75

able to be used for the rapid single-step detection of the quite high concentrations of pyrophosphate in real samples without any pretreatment and dilution of a sample. It has been shown that limits of detection are lower in case of silver nanoparticles compared to gold. The limit of detection can be decreased by decreasing concentration of nanoparticles. The procedure is quick, simple and can be easily realized as a semi-quantitative naked-eye test.

LOD, ␮g mL−1

AuNPs 20 45 70 AgNPs 6 12 16

Acknowledgments

3.3. Real sample analysis The method developed was used to determine pyrophosphate in “Dr. Oetker” baking powder. This powder consists of: sodium pyrophosphate, a pH regulator, sodium bicarbonate, and corn starch. The results of the pyrophosphate determination using developed method are shown in Table 2. They are consistent with those obtained by atomic emission spectroscopy and potentiometric titration, indicating good accuracy of the proposed method. Also the method was used for determination of pyrophosphate in copper plating solution, where it is added as a ligand to bind copper in the anion complexes to improve quality of the coating formed. The composition of the copper plating solution and the results of the pyrophosphate determination are given in Table 3. The data also show good accuracy of the proposed method. The method was used for determination of pyrophosphate in two spiked samples of bread. Composition of the samples of bread and results of the pyrophosphate determination are given in Table 4. The data also show good accuracy of the proposed method. 4. Conclusions It has been demonstrated that 6,6-ionene is a promising cationic polymer for synthesis of stable positively charged gold and silver nanoparticles, which is connected with their both electrostatic and steric stabilization by this polymer. These desensitized nanoparticles shown to have selectivity to pyrophosphate and sulfate, and

The reported study was funded by Russian Foundation for Basic Research and Moscow city Government according to the research project N 15-33-70002 mol a mos. We also thank MSU Joint Use Center and Dr. Sergey S. Abramchuk for recording TEM images and ED patterns of the samples, Dr. Elena N. Shapovalova and Anna N. Ioutsi for providing us with the 6,6-ionene samples. The authors acknowledge partial support from M.V.Lomonosov Moscow State University Program of Development. References [1] A. Jastrzebska, Capillary isotachophoresis as rapid method for determination of orthophosphates, pyrophosphates, tripolyphosphates and nitrites in food samples, J. Food Compos. Anal. 24 (2011) 1049–1056. [2] Z. Zhong, G. Li, Determination of phosphate, pyrophosphate, metaphosphate and total phosphorus in seafoods by ion chromatography, Chin. J. Chromatogr. 27 (2009) 499–504. [3] K.-C. Yang, C.-C. Wang, C.-C. Wu, T.-Y. Hung, H.-C. Chang, H.-K. Chang, F.-H. Lin, Acute and subacute oral toxicity tests of sintered dicalcium pyrophosphate on ovariectomized rats for osteoporosis treatment, Biomed. Eng.-Appl. Basis Commun. 22 (2010) 169–176. [4] C. Efstathiou, T. Hadjiioannou, Potentiometric titration of fluoride, sulfate, chromate, molybdate, tungstate, oxalate. Phosphate, pyrophosphate and hexacyanoferrate (II) ions with lead (II) solutions and a fluoride-selective electrode, Anal. Chim. Acta 109 (1979) 319–326. [5] Y. Lin, L. Hu, L. Li, K. Wang, Y. Ji, H. Zou, Electrochemical determination of pyrophosphate at nanomolar levels using a gold electrode covered with a cysteine nanofilm and based on competitive coordination of Cu(II) ion to cysteine and pyrophosphate, Microchim. Acta 182 (2015) 2069–2075. [6] B. Ya. Spivakov, T.A. Maryutina, L.K. Shpigun, V.M. Shkinev, Yu.A. Zolotov, Determination of ortho- and pyrophosphates in water by extraction chromatography and flow-injection analysis, Talanta 37 (1990) 889–894.

Table 2 Determination of pyrophosphate in “Dr. Oetker” baking powder (n = 3, P = 0.95). Method Proposed method

AuNPs AgNPs

Optical emission spectroscopy Potentiometric titration

Found, %

RSD, %

34.3 ± 1.4 34 ± 2 35.33 ± 0.07 36 ± 2

2 2 0.08 3

Table 3 Determination of pyrophosphate in the copper plating solution (n = 3, P = 0.95). The solution composition

Na4 P2 O7 280 mg mL

−1

CuSO4 ·5H2 O 70 mg mL

AuNPs

−1

NH4 NO3 10 mg mL

−1

AgNPs

Found, mg mL−1

RSD, %

Found, mg mL−1

RSD, %

272 ± 29

4

250 ± 30

5

Table 4 Determination of pyrophosphate in the bread samples using AgNPs (n = 3, P = 0.95). Bread composition

Spiked level, mg g−1

Found, mg g−1

Recovery, %

RSD, %

Loaf of “Bakery №22” company (baking wheat flour of top grade, water, sugar, sunflower oil, salt, baking yeast) Wheat sandwich bread “American Sandwich” of “Harris CIS” company (white flour of top grade, water, sugar, vegetable fat, baking improver [emulsifier (mono- and diglycerides of fatty acids); preservative − calcium propionate; white flour; soya flour; enzymes; antioxidant − ascorbic acid], food alcohol, salt, dry wheat gluten, yeast

5.2

4.9 ± 0.4

94.2

3.1

5.1

5.5 ± 0.4

107.8

2.4

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Biographies E.A. Terenteva is a PhD student at Chemistry Department of Lomonosov Moscow State University. Her study deals with gold and silver nanoparticles in development of optical SPR sensorss. V.V. Arkhipova received her PhD degree at Lomonosov Moscow State University in 2015. Her research interest includes gold nanoparticles and polyurethane foam based gold nanoparticle composites. V.V. Apyari received his PhD degree at Lomonosov Moscow State University in 2009. Now he is a senior scientist at Chemistry Department of Lomonosov MSU (Division of Analytical chemistry). His research interest includes optical sensors based on gold and silver nanoparticles, optical spectroscopy, and test-methods. P.A. Volkov is a researcher at Scientific-Research Institute of Chemical Reagents and Special Purity Chemicals. His research deals with spectroscopy, and nanoparticle characterization. S.G. Dmitrienko is a professor at Lomonosov Moscow State University. Her research interest includes solid-phase extraction, magnetic SPE sorption preconcentration, nanotechnology in analytical science, optical sensors.