Strong hydrogen bonding of gallic acid during synthesis of an efficient

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Strong hydrogen bonding of gallic acid during synthesis of an efficient AgNPs colorimetric sensor for melamine detection via dis-synthesis strategy Maryam Farrokhnia, Sadegh Karimi, and Sahar Askarian ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05785 • Publication Date (Web): 24 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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ACS Sustainable Chemistry & Engineering

Strong hydrogen bonding of gallic acid during synthesis of an efficient AgNPs colorimetric sensor for melamine detection via dis-synthesis strategy Maryam Farrokhnia*a, Sadegh Karimi*b and Sahar Askarianb a

The Persian Gulf Marine Biotechnology Research Center, The Persian Gulf Biomedical Sciences Research Institute, Bushehr University of Medical Sciences, Iran b

Department of Chemistry, College of Science, Persian Gulf University, Bushehr, Iran b Oil and Gas Research Center, Persian Gulf University, Bushehr 75169, Iran

_____________________________________________________________________ ABSTRACT We developed a convenient, label free and non-aggregation–based silver nanoparticles probe for colorimetric detection of melamine. Ag nanoparticles (AgNPs) were synthesized using gallic acid, known as 3, 4, 5-trihydroxybenzoic acid, as a reducer and stabilizing agent at room temperature simultaneously. Formation of AgNPs was confirmed by various technical analysis e.g, UV–vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), energy dispersive spectroscopy (EDS), elemental mapping, powder X-ray diffraction (XRD), X-ray Photon Spectroscopy (XPS), dynamic light scattering (DLS) and High-resolution transmission electron microscopy (HRTEM). Since the gallic acid has three hydroxyl groups, it can interact with melamine via several hydrogen bonds which has been confirmed with theoretical study by density functional theory. So that, in the presence of melamine, the free gallic acid as a responsible agent for synthesis of AgNPs, is decreased. Consequently, the formation of silver nanoparticles is disrupted because reducers aren't enough for reduction of Ag+ ion. Simultaneously, the color and spectral changes of AgNPs depend on melamine concentration such that lower melamine concentration is equivalent to yellow (high absorbance) and higher concentration corresponds with colorless solution (low absorbance). In this way, the Plasmon absorbance of AgNPs at 400nm can be used for quantitative measurement of melamine. Our finding indicated that sensitive linear calibration curve can be obtained between absorbance at 400 nm and logarithm of melamine concentration in the range of 0.04-20 μM with linear coefficient 0.992. The assay conveys detection limit 3.609 nM ±0.014 (3σ) and shows high potential ability for melamine detection in a selectivity study.

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Corresponding authors, E-mail address: karimi.sadegh@ gmail.com; [email protected] Maryam Farrokhnia: [email protected]

Keywords: Melamine, Gallic acid, Ag nanoparticles, Hydrogen bond, Density functional theory

Introduction Melamine (1,3,5-Triazine-2,4,6-triamine) toxicity causes a serious threat to human health particularly to kidneys and the reproductive system. It even could result in death especially for vulnerable individuals such as infants and young children1-2. Although, melamine is highly nitrogen rich heterocyclic (66% by mass) mainly used to produce melamineformaldehyde resins, it is also known for its illegally adulteration of milk, infant formula, poultry and other animal feeds to increase their apparent protein contents measured by analyzing their nitrogen contents. Hence, the safety regulation level (2.5 ppm for non-infantformula , 1 ppm threshold in infant milk products in USA, EU and 1ppm in China) has been established3 and more adulterations can cause serious health problems4. Consequently, detection of melamine in food sources is a topic of many discussions. Some available methods have been applied for detection of melamine in food products such as gas chromatography GC5, high performance liquid chromatography/mass spectrum (HPLC/MS)67

, surface enhanced Raman spectroscopy8, capillary zone electrophoresis/mass spectrum

(CE/MS)9-10 and micellar liquid chromatography (MLC)11. Although all these methods are highly specific and sensitive, they are also expensive, time consuming and require extensive sample preparation in addition to their urgent need to highly skilled personnel. Thus, there have been increasing demands for developing alternate methods which can overcome challenges or at least minimize these problems while they are highly sensitive and convenient at the same time. On the other hands, advances in basic sciences (chemistry, physics, molecular biology and biochemistry) have mainly resulted in sensor development of important elements with much greater selective and sensitive assay. Moreover, the recent advancements in nanotechnology application have introduced it as a multi-dimensional tool. On other word, the exclusive optical properties of noble metals nanoparticles namely silver (Ag)12 and gold (Au)13, have extremely revolutionized the colorimetric sensing of elements. So far, nanotechnology has shown its great potential for rapid and high throughput detection of melamine while it needs only small amount of sample. Gold and silver nanoparticles have 2 ACS Paragon Plus Environment

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been applied as colorimetric sensors because of their interesting physical and chemical properties. The induced aggregations of Au and Ag nanoparticles by analyte have been used in designing sensors for various compounds with biological or chemical importance. But up to now, there are limited studies based on the colorimetric detection of melamine by Ag and AuNPs aggregations, which are induced by inter-particle crosslinking14-27 3, 28-34. Melamine is a very good candidate for hydrogen bonding (H-bond) interactions with molecule has H-bond donor or acceptor groups such as carboxyl and phenolic-OH group. Hence, we have designed a one-step synthesis of AgNPs by naturally occurring molecule namely gallic acid as both reducing and capping agent35. Since, gallic acid has three phenolic–OH and one carboxyl group; can be a very good promising candidate for hydrogen bonding with melamine. This H-bond interaction leads to a decrease of free reducers in the solution and results in interruption of AgNPs formation consequently In the present study, this one-step green synthesis will offer selective, sensitive, fast and convenient colorimetric detection of melamine at ambient temperature by using the fact that melamine-gallic acid intermolecular H-bonding can cause to a decrease of AgNPs formation. To the best of our knowledge this is the first report using gallic acid assay for sensing of melamine.

Experimental section Material and reagent Silver nitrate (AgNO3), NaOH, hydrochloric acid (HCl), gallic acid, Melamine·, L-arginine, L-alanine, L-phenylalanine , L-valine , L-serine , L-leucine , L-isoleucine , L-Tryptophan, Ltyrosine, glycine, urea and vitamine-B6 were obtained from Sigma-Aldrich (St Louis, MO). All the chemical reagents were of analytical grade and used directly without any further purifications. Moreover, they were stored at 4 °C in a refrigerator until use.

Instrumentation UV−vis absorption spectra were measured on Specord Analytic Jena UV–visible spectrophotometer. The infrared spectra of melamine and produced nanoparticles were measured in KBr pellets using FTIR spectroscopy measurement (BrukerVector22 spectrometer) in the transmittance mode at a resolution of 4 cm−1. In order to investigate the dispersion of the element in the AgNPs solution, energy dispersive spectroscopy (EDS)



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elemental mapping image was carried out by JEOL, JSM-7610F Ltd. A dynamic light scattering (DLS) particle size analyzer (Horiba LB-550) was used to determine the distribution of the particle size. High Resolution Transmission Electron Spectroscopy (HRTEM) of Ag-NPs were obtained before and after interaction with melamine by a JEOL, JEM-2100F, 200 KV TEM. X-ray diffraction (XRD) measurements were conducted on a D8, Avance, Bruker, AXS diffractometer with Cu-Kα radiation (λ= 1.5418 Å) with a scan speed of 2◦/min. X-ray photoelectron spectroscopy (XPS) was performed by Thermo Scientific, ESCALAB 250Xi instrument with Mg Kα radiation as the X-ray source in order to determine the chemical composition and the valence states of the prepared nanoparticles. It should be mentioned that all the glasswares in the proposed assay were cleaned by 3:1 (v/v) HCl-HNO3, and dried in oven.

Computational details The electronic study of all investigated compounds was performed at the density functional theory (DFT) level. The B3LYP hybrid functional36-37in conjunction with Popel basis set 6311++g(2d,2p) were applied to the systems. It has been reported that this functional is one of the most popular ones in the study of hydrogen bonds38-40. Full geometry optimizations of all molecules were carried out without symmetry constrain. The minima on the potential energy surfaces were qualified by the absence of negative eigenvalues in the diagonalized Hessian matrix, giving imaginary normal vibrational mode. The electron density, 𝜌 , its Laplacian,∇2 𝜌 and potential energy V at the bond critical points (BCP) were computed based on Bader’s quantum theory of atoms in molecules41 by AIM2000 package

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. The solvent effect on the

electronic properties in addition to intermolecular interaction energies were evaluated by employing the self-consistent reaction field (SCRF) method with the (SMD) solvation model43. Water was selected as a solvent for study. All calculations were done by Gaussian 09 suite of program44.

Results and discussions Preparation of gallic stabilized AgNPs Silver nanoparticles were synthesized according to the Park et.al procedure

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by

reducing Ag+1 using naturally occurring molecule, gallic acid. The redox potential of gallic acid and Ag+1 ion are 0.5 and 0.8 V respectively confirmed the thermodynamically favorable reduction of Ag+1 at room temperature.


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Simply, 3.960 mL of gallic acid (5μM) was added into 10 μL of AgNO3 (40mM) under vigorous stirring and then the final volume of the sample tube was arranged to 4mL using deionized water. During the synthesis of AgNPs, accurate pH adjustment of solutions have been carried out at pre-specific value, 4.5-9.0, by a digital pH meter Metrohm with a combined glass electrode. According to the obtained results in Figure S1a, in supplementary section, at the pH= 7.0 there is a maximum production of AgNPs and no sensible change has been observed in the absorbance at 400 nm. It should be mentioned that, similar to Park et al procedures

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, the prepared mixture was vortexed for 5s. However, in contrast to his

procedures, the incubation time was 120 minutes at ambient temperature condition (25 °C). After the desired time, the color of solution turned from colorless to yellow, indicating that the synthesis of silver nanoparticles has been done.

Time, pH and gallic acid concentration optimization of AgNPs synthesis

Time: in order to optimize the time of nanoparticles synthesis, the kinetic of AgNPs formation was investigated at pH=7.0 during reaction with gallic acid. In this way, the absorption spectrum of reaction mixture was recorded in the 15 min intervals (Figure 1a).The kinetic profile at 400nm has been depicted in Figure 1b, contains some interesting information. It is clear in Figure 1a&b that a rapid growth with time is evident from the absorption at 400 nm at the early stage of synthesis but absorbance slowly increases over the next 100 minutes. In other words, we will see a peak at 400 nm passing the first 15 minutes right after reaction launches. This evidence is in agreement with previous publications used resembling structures such as dihydroxybenzoic acid (DBA) as a reducing agent for AuNPs synthesis32. The process, as an autocatalytic type, includes three stages namely nucleation, growth and saturation. Competition between first two stages, rate of nucleation and growth is determined by the size of nanoparticles. If the rate of nucleation is much higher than the growth step, similar to our case, we will be faced with a large number of small nuclei. In this condition, gallic acid reduces the most amounts of Ag+ ions to Ag (0) in the nucleation period and then these small nuclei will agglomerate to build the nanoparticles. Gradually, produced silver nanoparticles increase the intense of surface Plasmon band. Based on the obtained results, 120 minutes for reaction time was selected as the optimum ones, when the absorbance of AgNPs would reach to a plateau level.



pH: The structure and redox reaction of gallic acid reported in scheme 1A. This is apparent that H+ ions have critical roles in the redox reaction hence this rationale encouraged us to investigate the pH influence on AgNPs synthesis. Since the goal of our study is to introduce the new colorimetric sensor for melamine, the pH optimization has been done in the presence of melamine. It is worthy to mention that thanks to reducing power of gallic acid the AgNPs were synthetized. So that limiting access of silver nitrate salt to gallic acid led to a decrease in the synthesis of AgNPs. Therefore, gallic acid was firstly added to melamine (both with 5µM concentration). After 60 minutes, silver nitrate (0.1 mM) was entered to previous solution and then the pH of obtained solution was adjusted and then determined by a digital pH meter Metrohm with a combined glass electrode. According to the Figure (S1b) the most color changes are seen at pH=7.0. The possible reason can be due to the roles of hydroxyl and carboxylic groups of gallic acid and H+ in the its reduction-oxidation process. Yoosaf et.al,46 have been investigated their roles in the synthesis of AuNPs carefully. It should be mentioned that nanoparticles stability is very important factor in the synthesis route by gallic acid and their derivatives. Importantly, Yoosaf et.al stated that as-prepared nanoparticles immediately precipitate in the absence of any stabilizing reagent. So that for avoiding this, pH should be adjusted. On the other hand, according to the pKa of gallic acid (COOH: 4.40, OH: 9.88), it exists in ionized form at pH=7.0 which is also enhanced the electrostatic interaction with the nanoparticle surface. Thus the best adjusted pH is 7.0 when the dihydroxyl groups underwent two-electron oxidation reaction (scheme 1A) at proposed pH and produced its corresponding quinone form. Then the as-synthesis nanoparticles were stabilized through the interaction of negative carboxylic acid group (scheme 1B). Hereafter, pH=7.0 was selected as an optimum for our colorimetric experiments. Gallic acid concentration: As we have brought it up in the previous section, the synthesis of AgNPs has been launched using reducing power of gallic acid while it can be applied as stabilizing agent because of its bifunctional groups at the same time. As the Yoosaf et al46 and other research groups45identified, gallic acid has been undergone the two electron oxidation process (scheme 1A) and its available concentration is very vital for synthesis of stable nanoparticles. Obtained results in Figure (2a) uncover that, depending upon the concentration of gallic acid; the color and spectrum of obtained AgNPs are different. Our results have been specified that the concentration of gallic acid would affect on the absorbance and Plasmon peak position of AgNPs which is in agreement with Park et.al findings45. Since this designed sensor will be used for determination of melamine, the optimization of gallic acid concentration was done in the presence of melamine. As a rule, at 6 ACS Paragon Plus Environment

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low concentration (

Strong hydrogen bonding of gallic acid during synthesis of an efficient

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