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Synthesis and characterization of new tyrosine capped anisotropic silver nanoparticles and their exploitation for the selective determination of iodide ions
Accepted Manuscript Title: Synthesis and Characterization of new Tyrosine Capped Anisotropic Silver Nanoparticles and their Exploitation for the Selective Determination of Iodide Ions Authors: Annalinda Contino, Giuseppe Maccarrone, Massimo Zimbone, Mimimorena Seggio, Paolo Musumeci, Alessandro Giuffrida, Lucia Calcagno PII: DOI: Reference:
S0927-7757(17)30503-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.05.056 COLSUA 21649
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
23-3-2017 13-5-2017 20-5-2017
Please cite this article as: Annalinda Contino, Giuseppe Maccarrone, Massimo Zimbone, Mimimorena Seggio, Paolo Musumeci, Alessandro Giuffrida, Lucia Calcagno, Synthesis and Characterization of new Tyrosine Capped Anisotropic Silver Nanoparticles and their Exploitation for the Selective Determination of Iodide Ions, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.05.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Characterization of new Tyrosine Capped Anisotropic Silver Nanoparticles and their Exploitation for the Selective Determination of Iodide Ions. Annalinda Contino,*[a] Giuseppe Maccarrone,[a] Massimo Zimbone,[b] Mimimorena Seggio,[a] Paolo Musumeci,[c] Alessandro Giuffrida[a] and Lucia Calcagno.[c] [a]
Dipartimento di Scienze Chimiche Università degli Studi di Catania, Viale A. Doria 6, 95125 Catania, Italy Fax: (+) 39 (0) 95 580138 e-mail: [email protected] [b] CNR‐IMM, MATIS via S. Sofia 64, 95123 Catania, Italy [c]
Dipartimento di Fisica e Astronomia Università degli Studi di Catania, Via S. Sofia 64, 95123 Catania, Italy. Dedicated to the memory of Carmela Spatafora
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Highlights New anisotropic silver nanoparticles (AgNPs) have been synthesized and capped with L‐Tyrosine. The AgNPs have been characterized by a combined multi‐technique approach (TEM, UV‐Vis Spectroscopy, DLS and DDLS). The synthesized nanoparticles have been exploited for the selective determination of iodide in the presence of chloride ions.
Abstract. The assessment of the iodine status of natural waters is crucial for focusing future strategies for controlling and monitoring iodine deficiency disorders (IDD). Nanoparticles have been increasingly used as sensors for several organic and inorganic analytes. In this study, we report the synthesis of new Tyrosine Capped Anisotropic Silver Nanoparticles (AgNPs). The AgNPs were characterized by TEM, UV–Vis spectroscopy, and polarized and depolarized dynamic light scattering measurements, and were used for the quantitative determination of iodide ions in the presence of excess chloride ions. Both anions gave rise to an etching of the tips of the nanoprims converting them in rounded nanoplates. However, iodide ions perform this etching much better than chloride ones, allowing for their selective determination in tap waters. Fluoride and bromide anions give rise to a more efficient etching than chloride ions, but their presence does not interfere with iodide determination. This method was also used to determine the concentration of iodide in a sample of drinking water. Keywords: Anisotropic Silver Nanoparticles; L‐Tyrosine; Iodide; Dynamic Light Scattering. Introduction Iodine is the least abundant stable halogen and the heaviest essential element. Iodine is found in thyroid hormones and is a crucial micronutrient that is required for the synthesis of the main thyroid hormones. Iodine deficiency causes several severe disorders, such as endemic goitre, as well as brain damage and intellectual disability (endemic cretinism) and it is thus considered a worldwide health problem. Consequently, the assessment of the iodine status of a population is a crucial point to focus the appropriate strategies for controlling and monitoring iodine deficiency disorders (IDD), as well as the potential side effects of excessive iodine intake.1 In fact, iodine, mainly as iodide ions, can be found in tap waters with concentrations spanning in the range 3‐54 µg per litre in UK ground waters,2 or in a range <1.0 to 139 µg/l in Denmark,3 to more than 300 µg in some areas of China,4,5 probably due to particular geological factors.6 Thus, there is a strict correlation between high iodine level in drinking‐water and goitre prevalence, that indicates the necessity of stopping the provision of iodised salt in these areas, as well as caution against a total daily iodine intake that exceeds 800 µg/d.7 Therefore, it is crucial to determine the iodide content of drinking water to establish the amount of supplementation required and the root cause of both insufficient and excessive iodine intake. Several methods have been used for the determination of iodide concentration in waters, such as ion chromatography8,electrostatic ion chromatography,9 gas‐chromatography with mass spectrometry detection,10 microextraction techniques combined with spectrophotometry,11 stripping voltammetry,12 capillary electrophoresis,13 indirect atomic absorption spectrometry14, and spectrofluorimetric methods.15 In certain cases these methods have a very low limit of detection (LOD) and wide dynamic linear ranges. Therefore, they require complicated, multistep sample preparation, sophisticated instrumentation and/or accurate simulations16 that do not meet the requirements of a quick determination method for potentially unhealthy threshold values. Noble metal nanoparticles have unique properties, such as size‐ and shape‐dependent optical and electronic features, a high surface area to volume ratio, surfaces that can be readily modified, as well as very large extinction coefficients,17 therefore have many applications as colorimetric sensors for many analytes. However, although a reasonable understanding of the relationship between the properties of a particle and its size and composition has been developed for Au and Ag, the research on the relationship between the shape of a nanoparticle and its physical and chemical properties has
started more recently.18,19,20,21 In particular, triangular silver nanoprims have remarkable optical properties. For example, the peak extinction wavelength (λmax), of their localized surface plasmon resonance (LSPR) spectrum is unexpectedly sensitive to nanoparticle size, shape, and local (~10‐30 nm) external dielectric environment, allowing development of a new class of nanoscale affinity sensors and biosensors.22,23 In particular, halide ions can trigger the oxidative etching of anisotropic noble nanoparticles, with iodide ions being the most efficient,24 and thus dramatically influence the optical properties of silver nanoprisms, making them an ideal candidate for the detection and quantification of iodide ions.25,26 In this paper we report a study on anisotropic L‐tyrosine capped silver nanoparticles (AgNPs) obtained by a new synthetic route that allows binding the un‐oxidized tyrosine on the nanoparticles surface. Even though amino acids have been used as capping agents for spherical nanoparticles, allowing the obtainment of very stable colloids with multiple properties,27,28,29,30 and short peptides have been used to replace the poly(vinylpyrrolidone) (PVP) on the surface of silver nanocubes,31 this is the first example, at the best of our knowledge, of amino acid capped anisotropic silver nanoparticles. These new nanoprims have been carefully characterized by a combined approach which implements the use of a solid state technique (TEM), as well as UV‐Vis spectroscopy and polarized and depolarized Dynamic Light Scattering (DLS and DDLS) in solution and their optical properties have been exploited for the quantitative determination of iodide ions in the presence of a large excess of chloride ([Cl‐]/[I‐] ≤ 1000). This selectivity is of crucial importance for practical applications. Chloride, in fact, is always present in tap waters, even if chloride levels in unpolluted waters are often below 10 mg/litre.32 The determination of iodide ions was also carried out in the presence of fluoride and bromide at the concentrations they are typically present in drinking water,32,33 resulting selective as well. This approach would be easily extendible to develop simple, rapid and sensitive assays for the determination of iodide concentration in tap waters, quickly assessing the necessity and the amount of iodide supplementation. Experimental part. 2.1. Materials. L‐Tyrosine (L‐Tyr), sodium fluoride (NaF), sodium chloride (NaCl), sodium bromide (NaBr) and potassium iodide (KI) were obtained as commercial reagents by Merck and were used as received. Silver nitrate (AgNO3), sodium borohydride (NaBH4), hydrogen peroxide (H2O2) and trisodium citrate (Na3Cit.2H2O) were purchased from Sigma Aldrich. 2.2. Synthesis of AgNPs. The L‐Tyrosine capped silver nanoparticles (L‐Tyr AgNPs) were synthesized by slightly modifying the procedures previously reported for citrate capped triangular AgNPs.34,35 To summarise, silver nitrate (0.1 M, 25 µL), L‐Tyrosine (2 mM, 2.06 ml), hydrogen peroxide (30 wt%, 60 µL) and sodium borohydride (100 µl 0.1 M) were successively added to 24.75 mL of water continuously magnetically stirred in the presence of air. The solution turned pale yellow and slightly cloudy. 0.022 gr of Na3Cit.2H2O and 100 µL of hydrogen peroxide were subsequently added and the solution became colourless. A further addition of NaBH4 gave rise to a solution colour change from colourless to red, green, and blue. 2.3. TEM analysis. Transmission electron microscopy was performed with a TEM JEOL JEM 2010 F, equipped with the Gatan imaging filter, operating at 200 keV. Samples for TEM were prepared by placing several drops of the colloidal dispersions of AgNPs onto a carbon‐coated copper micro‐grid (2 M STRUMENTI).
2.4. DLS measurements.
The Dynamic Light Scattering (DLS) measurements were carried out by a homemade apparatus as described elsewhere.36 The sample was lighted with a 633 nm He–Ne laser whose power ranged between 15 and 150 mW. The incident light was vertically polarized while the polarized (DLS) and depolarized (DDLS) scattering intensities were detected using a polarizer, positioned between the sample and the detector, enabling selective collection of either the vertically polarized scattering, VV, and the horizontally depolarized scattering, VH. The autocorrelation functions were measured at 90° corresponding to q=18 µm-1. The analysis of the fluctuations (due to the Brownian motion) of the scattered light was performed by the intensity auto‐correlation function (g2) defined as:
,
1 1 lim → 〈 〉
,
,
1
(1)
where I is the scattered light intensity, is the temporal average scattered light intensity, T is the acquisition time, q is the scattering vector defined as q = (4πn/λ)sin(θ/2), n is the refraction index of the solvent, λ is the light wavelength, and θ is the scattering angle. The field autocorrelation function g1 is computed from the g2 using the Siegert relation (g2=|g1|2 ).37 However, for spherical nanoparticles in Brownian motion the g1 function is a decreasing exponential with a relaxation rate Γ (Γ=1/ with τ = decay time), whereas for solutions containing anisotropic nanoparticles the g1 function shows several exponential decay components that were thus analysed either by cumulant or multi‐exponential analysis.38 From the vertically polarized scattering relaxation rate (ΓVV), it is possible to calculate either the translational diffusion coefficient (Dt) and the hydrodynamic radius (RH) using the following expressions:
ΓVV = Dtq2
6
(2) (3)
where k is the Boltzmann constant, T is the absolute temperature and is the water viscosity. The hydrodynamic radius is the radius of a hypothetic sphere that diffuses with the same Dt of the nanoparticle under examination. Analogously, the rotational diffusion coefficient can be determined by VH through the relation:
ΓVH = Dtq2 + 6Dr
(4)
The relations (2) and (4) allow evaluation of the translational and rotational diffusion coefficients, assuming a decoupling of rotational and translational diffusion mechanisms. We have already reported that this technique is useful to characterize plasmonic anisotropic nanoparticles, such as gold nano‐rods, nano‐clusters and silver nanoplatelets.36
2.5. UV–Vis spectroscopic measurements. UV–Vis spectra of the investigated systems were carried out on the undiluted silver sol at room temperature, using a diode‐array Agilent 8453 spectrophotometer in the 200–1100 nm wavelength range. The effect of F‐, Cl‐, Br‐ and I‐ ions on the UV–Vis spectrum of the triangular L‐Tyr capped AgNPs was investigated by adding appropriate amounts of each ion solutions (10‐5, 10‐4, 10‐3, 10‐2, 10‐1 or 1 M) to 2.0 mL of the silver sol, reaching a final concentration range of 2 x 10‐6‐ 7 x10‐5 for fluoride, 1 x 10‐6‐1 x 10‐3 M for chloride, 1 x 10‐6 ‐ 2.5 x 10‐5 for bromide and 1 x 10‐7 ‐ 5 x 10‐6 M for iodide, respectively. The determination of iodide was also carried out at a fixed concentration of the other halide ions to test the selectivity of the assay. Furthermore, in order to determine the I‐ concentration in a drinking water, a 0.200 ml sample of water was mixed with 1.800 ml of silver nanoparticles and was spiked with a iodide solution to obtain final I‐ concentrations ranging from 2.5 x 10‐7 M to 5 x 10‐6 M. UV‐Vis spectra were carried out on the resulting solutions.
Results and Discussion 3.1 Synthesis and characterization of L‐Tyr capped triangular AgNPs. 3.1.1 The synthesis of the triangular L‐Tyr capped nanoparticles was carried out by following the method reported in the reference 34, that overcomes the photoinduced39,40 and seed‐mediated41 syntheses. Yin et al. in fact, exploited the dynamic equilibrium between the reduction of silver ions by NaBH4 and the oxidative etching of metallic silver by H2O2. Furthermore, citrate, or other many di‐ and tricarboxylate compounds, (whose two nearest carboxylate groups are separated with two or three carbon atoms), if used in the appropriate ratio, play a fundamental role as capping agents, giving rise to well defined nanoplates. In this work, to obtain the triangular L‐Tyr AgNPs, L‐Tyr was added to the solution reaction in order to promote an additional capping. Tyrosine, in fact, is widely used as capping agent, either in the oxidized42 and in the not oxidized form,29 being able to bind the silver nanoparticles surface by the semiquinone group or the O‐ terminal group, respectively. 3.1.2 TEM The TEM images of the L‐Tyr AgNPs, reported in Figure 1a and 1b, clearly show the formation of few spherical nanoparticles and many nanoplate structures, most with a triangular (even if snipped) shape that strictly resembles a pick, i.e. a guitar plectrum. Stacks of particles, easily found on the film (Figure 1c), allowed an estimation of their thickness (~8 nm). The histogram in Figure 1d represents the Feret diameter distribution of the nanoprisms, that results, as average, 40 ± 5 nm. 3.1.3 UV‐Vis Spectroscopy. In Figure 2 the UV‐Vis spectra of the L‐Tyr AgNPs immediately after preparation, and after two weeks and three months of aging are reported. Even though the silver sol was stored in the dark to avoid photo‐induced effects on the shape and the geometrical characteristics of the nanoparticles, Figure 2 shows a temporal evolution in the recorded spectra, indicating evident aging occurring nonetheless. However, the spectra recorded after one and three months are similar showing that after around 30 days the sol is stable. The spectra show the typical characteristics reported in the literature for triangular nanoprisms,43,44 i.e. a strong dipole resonance at 636 nm, a weaker in‐plane quadrupole resonance at 433 nm, as well as a weak transversal out‐of‐ plane quadrupole band (334 nm). Indeed, the band for the dipole resonance was initially centred at λmax = 650 nm and gradually blue‐shifted to λmax = 636 nm, strongly indicating that the aging gave rise to a gradual snipping of the tips of the nanoprisms, as predicted by the theory of nanoparticle optical properties,43 the discrete dipole approximation (DDA) method45 and experimentally confirmed.46,47,48 In order to estimate the dimensions of the nanoparticles from the UV‐Vis spectra it should be considered that DDA calculations show that different particle thickness, in combination with modest differences in edge length, account for large differences in the UV‐Vis spectra of the nanoprisms colloids.49 Thus, from the UV‐Vis measurements an approximate value of 60 nm for the particles size was obtained,50 that agreed with the TEM results. Nevertheless, an accurate determination of the nanoparticles size requires a more in‐depth study of their shape, that is achievable by the DLS and DDLS measurements. 3.1.4. DLS. UV–Vis spectroscopy allows determination of several colloid properties, such as concentration, dimensions and shape of nanoparticles. However, this technique gives no dynamical information on the motion of the particles in solution. Furthermore, whereas standard DLS is widely used to determine the average NPs size, information about the shape can only be obtained by using a fast and accurate method such as depolarized dynamic light scattering.
In Figure 3 the normalized autocorrelation function g2,VV for L‐Tyr AgNPs is reported. The measured autocorrelation function displays an exponential trend that confirms the assumption of Brownian motion. The obtained value of Dt is 9.2x10‐8 cm2/s, corresponding to a value of dH= 51 ± 4 nm. This is in agreement with the dimensions determined by TEM and UV‐Vis spectroscopy. In the same Figure the normalized autocorrelation function g2,VH for L‐Tyr AgNPs, which shows a decreasing exponential with a relaxation rate VH =49487 s‐1, is also shown. Using the equation (4), we obtained a Dr value of 8247 s‐1. The high value of the depolarized scattered signal confirms the anisotropic shape of the nanoparticles. In fact, depolarized scattering intensity is proportional to the “optical anisotropy”, i.e. the difference between polarizability parallel and perpendicular to the principal symmetry axis), whereas in the case of spherically symmetrical structures the rotation around an axis does not involve optical anisotropy and therefore does not give a contribution to the depolarized DLS signal. Anisotropically shaped nanoparticles with two or three different orthogonal dimensions could be related to ellipsoidal shapes, either nanooblates and nanoprolates. In particular, oblate shapes, including nanoplatelets and nanodisks, are characterized by one short “thickness” and two more or less similar, larger “widths”. The two distinctly different dimensions give rise to different spectroscopic characteristics which strongly depend on the aspect ratio of the nanoparticles.51 Using the classical model of Perrin,52,53 as reported in a previous paper,54 it was possible to calculate the values of the major and minor axes (a and b) of the ellipsoid, by using the two independent measured values of Dt and Dr. The obtained values are a = 14 nm and b= 70 nm, with an aspect ratio =b/a of 5. Thus, the appropriate use of dynamic light scattering methods allows an accurate determination of the shape and the size of nanoparticles, showing several advantages with respect to TEM. In fact, whereas TEM gives information on the systems under investigation after the removal of the solvent, the DLS and DDLS allow a careful characterization of the same systems in solution. Furthermore, these techniques are less expensive and faster than microscopic techniques. 3.2. Halide ion‐sensory properties of pick silver nanoparticles. The UV‐Vis spectra of the pick silver nanoparticles at different concentrations of chloride ions are reported in Figure 4. The Figure clearly shows that in 0‐1000 µM range the shape of SPR peak did not change and the differences in the absorbance values were only due to the dilution effects (within the experimental error). On the other hand, increasing the Cl‐ ions concentration from 0.01 M to 0.10 M gave rise to a progressive and dramatic blue‐shift of the absorption peak that indicated, at first, a pronounced snipping of the tips of the pick nanoparticles until, in the end, they became discoidal, as previously reported.55 However, in the paper of W. Xu et al., the chloride ions readily attacked the silver atoms at the vertex area that were selectively etched, starting just from a concentration of 1000 µM. In our case, instead, rounding of the nanoprisms was only observed at a concentration of chloride ions of nearly 0.01 M. This different threshold in the chloride concentration was probably due to the better performance of tyrosine as capping agent, that limited the “sculpturing effect” of chloride ions in shape transformation from triangular to discal silver nanoplates55 Hence, the UV‐Vis spectrum of the nanoparticles was not very sensitive to the chloride ions, being capable of their detection only at concentrations above 1000 µM. The addition of iodide ions gave rise to a quite different trend. In fact, the facet‐selecting etching of the nanoprisms was driven by the chemical characteristics of the etching agents. As it is well known, halide ions are precipitant agents for the silver ions and, between the resulting insoluble compounds, AgI is much more insoluble than AgCl. Thus, iodide ions can react with the active silver atoms, dissociating them from the nanostructure56 and subtracting them from the equilibrium much better than the chloride ones. Furthermore, iodide is the most efficient halide ion for the oxidative
etching of anisotropic noble nanoparticles24. However, at our experimental conditions the basic pH of the solution probably rules out the oxidation of iodide in iodine, as reported for bromide57 and iodide58 in weak acid solutions. The data reported in Figure 5a show at first (until to 2.9 µM) a decrease in the absorption of the SPR peak, that is undoubtedly not ascribable to the dilution, but instead to a partial snipping of the tips of the AgNPs. The population of the nanoparticles initially absorbing at 636 nm decreased, whereas the corresponding signal of the resulting “rounded” plates was too low to be detected. Above 3 µM the in‐plane dipole plasmon resonance peak at 636 nm blue‐shifted and its absorbance dramatically decreased, indicating a more important etching and thus an additional snipping of the tips of the triangular nanoparticles by the iodide ions. This gave rise to the formation of nanoplates55 instead of spherical particles, as indicated by the absorption of the SPR peak centred at 504 nm.46 Above 5 µM, no significant changes in the UV‐Vis spectra were detected; this could indicate that the snipping process is complete and cannot go further and that all the pick nanoparticles have been transformed into oblate nanoplates. This trend is opposite to the results reported by Yang et al.26 In this paper, in fact, raising the concentration of iodide ions transformed the nanoprims into spherical particles, as testified by the disappearance of the absorption bands due to the quadrupole oscillations, still present instead in the L‐Tyr capped AgNPs. This opposite mechanism is probably due to the improved performance of tyrosine as capping agent; in fact, this amino acid prevents the direct action of iodide on the nanoparticles surface that could induce their fusion into spherical symmetry aggregates, favouring instead the etching action and the resulting snipping of the tips. The A636‐A317 of the silver nanoparticles at different concentrations of iodide ions are plotted in Figure 5b. At low concentrations, the absorbance values decreased linearly with an increase in the concentration of iodide up to 2.9 μM. However, when the I‐ concentrations were above 3 μM, a straight line with a more negative slope was observed. The change of the slope could be ascribed to the fact that the etching process critically depended on the concentration of iodide ions that induced a different morphology of the nanostructures.59,60 In fact, whereas at low iodide ion concentrations the nanoparticles still had a pick shape, and thus the tips, even if rounded, were still present, representing a preferential reaction site, above 3 μM, the snipping process was almost complete and the I‐ species interacted in the same way either on the edges of the particles and on their flat surfaces (see Figure 6), lowering the thickness of the nanoplates and consequently their symmetry and ultimately giving rise to an increase of the peak due to the oscillation of the in‐plane quadrupole. Furthermore, as reported in Figure 7, even in the presence of a concentration 1000 µM of Cl‐, the UV‐Vis spectra of the pick‐shaped silver nanoparticles showed the same trend as a result of the addition of increasing concentrations of iodide ions. Up to a concentration of iodide ions of 2.9 µM, in fact, the absorbance of the in‐plane dipole plasmon resonance peak at 636 nm decreased, whereas above 3 µM this peak blue‐shifted and its absorbance dramatically decreased (Figure 7a). The plot of the A636‐A317 as a function of the concentration of the iodide ions (Figure 7b) showed a break point also in this case, even if the slopes of the straight lines were minor than those reported in Figure 5 that are referred to the measurements carried out in the absence of chloride ions. Obviously, the presence of chloride ions adsorbed on the surface of the nanoparticles renders them less prone to the etching of iodide ions. We also tested the effect of fluoride and bromide ions at their typical upper limits in drinking water (i. e. 70 and 24 μM, respectively) on the spectral characteristics of the pick‐shaped silver nanoparticles and the results are shown in Figure 8a and 8b, respectively. Unlike what was found for the chloride ions, either fluoride and bromide, already from a concentration of few micromolar, gave
rise to a decrease of the SPR absorbance that was clearly not ascribable solely to the dilution effects. The effect is more pronounced for bromide ions that above 10 μM also gave rise to a blue shift of the plasmon resonance peak. The changes of the surface plasmon peak of the pick‐shaped silver nanoparticles at a concentration of 5 μM of the four halides, summarized in Figures 8c‐e, could be easily fathomable, as the effects on the SPR peak increased as the solubility products of the silver halides decreased, with the exception of the effect of the chloride ions, that could, at first sight, appear odd. Nevertheless, it should be considered that silver ions form complex species with Cl‐ ions only, and that these species could compete with the formation of insoluble silver halides. In fact, in order to obtain the etching of anisotropic L‐Tyr silver nanoparticles it was necessary to reach a concentration of chloride ions major than 1000 μM. In drinking water the average concentration of F‐ and Br‐ ions rarely exceeds 0.1 and 0.3 ppm, respectively32,33 and thus their interference with the determination of iodide was evaluated at this concentration. In both cases the plots of the A636‐A317 as a function of the concentration of the iodide ions (Figure 8g and 8h) show a break point at a value slightly lower than those observed in the measurements carried out in the absence of other halide ions (Figure 5), even if the slopes of the straight lines are lower. Obviously, as previously observed for the measurements carried out in the presence of chloride ions, fluoride and bromide ions adsorbed on the surface of the nanoparticles renders them less prone to the etching of iodide ions. In order to evaluate the robustness of the analytical method for the determination of iodide ions in drinking waters proposed here, a sample of an oligomineral water distributed in Italy was analysed. To achieve this, several experiments were carried out by mixing different volumes of the silver colloid and the drinking water. The optimal ratio of volumes was found to be 1.800/0.200 ml of silver sol/drinking water. In fact, at this ratio, it was possible to evaluate the effect of ionic strength of the water on the nanoparticles without diluting the solution significantly. The UV‐Vis spectra of the resulting solution spiked with different concentrations of iodide ions are reported in Figure 9. The data showed a trend that is very similar to that concerning the addition of iodide to the silver colloid alone (Figure 5a). In fact, it was possible to observe at first (until to 2.5 µM) a decrease in the absorption of the SPR peak, not ascribable to the dilution alone, but instead to a partial snipping of the tips of the AgNPs. Above 2.5 µM the in‐plane dipole plasmon resonance peak at 636 nm blue‐ shifted and its absorbance dramatically decreased, indicating a more important etching. Above 4 µM, no significant changes in the UV‐Vis spectra were detected; this could indicate that the snipping process was complete and could not go further, and that all the pick‐shaped nanoparticles were transformed into oblate nanoplates. The only difference between the two trends was the values at which there is a variation of the slope (3.0 µM for pure sol and 2.5 µM for the drinking water, respectively), and at which no significant changes in the UV‐Vis spectra were detected (5.0 µM for pure sol and 3.8 µM for the drinking water, respectively). Probably, even though the analysed water was a light mineral one (fixed residue 22 mg/litre), the dissolved electrolytes gave rise to a small, but detectable, etching effect that made the variation of slopes and the saturation effect appear at concentrations lower than those observed for the colloid alone. Conclusions. New L‐Tyr capped silver anisotropic nanoparticles were synthetized and accurately characterized. The presence of tyrosine as a capping agent gave rise to the formation of well stable pick‐shaped particles. The use of different techniques (TEM, UV–Vis spectroscopy and DLS) allowed us not only to fully characterize novel L‐Tyr capped AgNPs, that showed a pronounced anisotropy, but also to smartly exploit them for the quantitative determination of I‐ ions in solution, even in the presence of a large excess of chloride ions. Thus, pick‐shaped nanoparticles could be used to develop simple,
rapid and sensitive assays for the determination of iodide concentration in tap waters, quickly assessing the necessity and the amount of iodide supplementation. Acknowledgments. The University of Catania (FIR 2014, 9DD800) is gratefully acknowledged for the financial support. We acknowledge Salvatore Pannitteri for technical assistance. References 1 R. Doggui, J. El Atia, Iodine deficiency: Physiological, clinical and epidemiological features, and pre‐analytical considerations, Annales d’Endocrinologie, 76 (2015) 59–66. 2 W.M. Edmunds, P. Shand, P. Hart, R. S. Ward, The natural (baseline) quality of groundwater: a UK pilot study, Sci. Total Environ., 310 (2003) 25–35. 3 K M Pedersen, P Laurberg, S Nøhr, A Jørgensen and S Andersen, Iodine in drinking water varies by more than 100‐fold in Denmark. Importance for iodine content of infant formulas, Eur. J. Endocrinol. 140 (1999) 400– 403. 4 H. Shen, S. Liu, D. Sun, S. Zhang, X. Su, Y. Shen, Hepeng Han, Geographical distribution of drinking‐water with high iodine level and association between high iodine level in drinking‐water and goitre: a Chinese national investigation, Br. J. Nutr., 106 (2011) 243–247. 5 P. Li, L. Liu, H. Shen, Q. Jia, J. Wang, H. Zheng, J Ma, D. Zhou, S. Liu, X. Su, The Standard, Intervention Measures and Health Risk for High Water Iodine Areas, PLOSONE, 9 (2014) e89608. 6 J. Gao, Z. Zhang, Y. Hu, J. Bian, W. Jiang, X. Wang, L. Sun, Q. Jiang, Geographical Distribution Patterns of Iodine in Drinking‐Water and Its Associations with Geological Factors in Shandong Province, China, Int. J. Environ. Res. Public Health, 11 (2014) 5431‐5444. 7 Z. Sang, P. P. Wang, Z. Yao, J. Shen, B. Halfyard, L. Tan, N. Zhao, Y. Wu, S. Gao, J. Tan, J. Liu, Z. Chen, W. Zhang, Exploration of the safe upper level of iodine intake in euthyroid Chinese adults: a randomized double‐blind trial, Am. J. Clin. Nutr., 95 (2012) 367–73. 8 N. Gros, Ion Chromatographic Analyses of Sea Waters, Brines and Related Samples, Water, 5 (2013) 659‐676. 9 W. Hu,, P.‐J. Yang, K. Hasebe, P. R. Haddad, K. Tanaka, Rapid and direct determination of iodide in seawater by electrostatic ion chromatography, J. Chromatogr. A, 956 (2002) 103–107. 10
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0.2 0.0 -6 -6 0.00 1.00 2.00x10 2.00 - 3.004.00x10 4.00 5.00 [I ] M Figure 7. UV-Vis a) Spectra of anisotropic L-Tyr AgNPs as a function of various concentrations of I- ions in presence of 1000 µM [Cl-]. (b) Plot of A636-A317 vs the concentration of the iodide ions.
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Figure 8. a)-f) UV-Vis spectra of anisotropic L-Tyr capped AgNPs at different concentrations of halide ions; plot of A636A317 vs the concentration of the iodide ions in the presence of g) fluoride and h) bromide ions.
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Figure 9. a) UV-Vis spectra of anisotropic L-Tyr capped AgNPs mixed with a sample of drinking water and spiked with different concentrations of iodide ions; b) corresponding plot of A636-A317 vs the concentration of the iodide ions.
S0927-7757(17)30503-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.05.056 COLSUA 21649
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
23-3-2017 13-5-2017 20-5-2017
Please cite this article as: Annalinda Contino, Giuseppe Maccarrone, Massimo Zimbone, Mimimorena Seggio, Paolo Musumeci, Alessandro Giuffrida, Lucia Calcagno, Synthesis and Characterization of new Tyrosine Capped Anisotropic Silver Nanoparticles and their Exploitation for the Selective Determination of Iodide Ions, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.05.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Characterization of new Tyrosine Capped Anisotropic Silver Nanoparticles and their Exploitation for the Selective Determination of Iodide Ions. Annalinda Contino,*[a] Giuseppe Maccarrone,[a] Massimo Zimbone,[b] Mimimorena Seggio,[a] Paolo Musumeci,[c] Alessandro Giuffrida[a] and Lucia Calcagno.[c] [a]
Dipartimento di Scienze Chimiche Università degli Studi di Catania, Viale A. Doria 6, 95125 Catania, Italy Fax: (+) 39 (0) 95 580138 e-mail: [email protected] [b] CNR‐IMM, MATIS via S. Sofia 64, 95123 Catania, Italy [c]
Dipartimento di Fisica e Astronomia Università degli Studi di Catania, Via S. Sofia 64, 95123 Catania, Italy. Dedicated to the memory of Carmela Spatafora
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Highlights New anisotropic silver nanoparticles (AgNPs) have been synthesized and capped with L‐Tyrosine. The AgNPs have been characterized by a combined multi‐technique approach (TEM, UV‐Vis Spectroscopy, DLS and DDLS). The synthesized nanoparticles have been exploited for the selective determination of iodide in the presence of chloride ions.
Abstract. The assessment of the iodine status of natural waters is crucial for focusing future strategies for controlling and monitoring iodine deficiency disorders (IDD). Nanoparticles have been increasingly used as sensors for several organic and inorganic analytes. In this study, we report the synthesis of new Tyrosine Capped Anisotropic Silver Nanoparticles (AgNPs). The AgNPs were characterized by TEM, UV–Vis spectroscopy, and polarized and depolarized dynamic light scattering measurements, and were used for the quantitative determination of iodide ions in the presence of excess chloride ions. Both anions gave rise to an etching of the tips of the nanoprims converting them in rounded nanoplates. However, iodide ions perform this etching much better than chloride ones, allowing for their selective determination in tap waters. Fluoride and bromide anions give rise to a more efficient etching than chloride ions, but their presence does not interfere with iodide determination. This method was also used to determine the concentration of iodide in a sample of drinking water. Keywords: Anisotropic Silver Nanoparticles; L‐Tyrosine; Iodide; Dynamic Light Scattering. Introduction Iodine is the least abundant stable halogen and the heaviest essential element. Iodine is found in thyroid hormones and is a crucial micronutrient that is required for the synthesis of the main thyroid hormones. Iodine deficiency causes several severe disorders, such as endemic goitre, as well as brain damage and intellectual disability (endemic cretinism) and it is thus considered a worldwide health problem. Consequently, the assessment of the iodine status of a population is a crucial point to focus the appropriate strategies for controlling and monitoring iodine deficiency disorders (IDD), as well as the potential side effects of excessive iodine intake.1 In fact, iodine, mainly as iodide ions, can be found in tap waters with concentrations spanning in the range 3‐54 µg per litre in UK ground waters,2 or in a range <1.0 to 139 µg/l in Denmark,3 to more than 300 µg in some areas of China,4,5 probably due to particular geological factors.6 Thus, there is a strict correlation between high iodine level in drinking‐water and goitre prevalence, that indicates the necessity of stopping the provision of iodised salt in these areas, as well as caution against a total daily iodine intake that exceeds 800 µg/d.7 Therefore, it is crucial to determine the iodide content of drinking water to establish the amount of supplementation required and the root cause of both insufficient and excessive iodine intake. Several methods have been used for the determination of iodide concentration in waters, such as ion chromatography8,electrostatic ion chromatography,9 gas‐chromatography with mass spectrometry detection,10 microextraction techniques combined with spectrophotometry,11 stripping voltammetry,12 capillary electrophoresis,13 indirect atomic absorption spectrometry14, and spectrofluorimetric methods.15 In certain cases these methods have a very low limit of detection (LOD) and wide dynamic linear ranges. Therefore, they require complicated, multistep sample preparation, sophisticated instrumentation and/or accurate simulations16 that do not meet the requirements of a quick determination method for potentially unhealthy threshold values. Noble metal nanoparticles have unique properties, such as size‐ and shape‐dependent optical and electronic features, a high surface area to volume ratio, surfaces that can be readily modified, as well as very large extinction coefficients,17 therefore have many applications as colorimetric sensors for many analytes. However, although a reasonable understanding of the relationship between the properties of a particle and its size and composition has been developed for Au and Ag, the research on the relationship between the shape of a nanoparticle and its physical and chemical properties has
started more recently.18,19,20,21 In particular, triangular silver nanoprims have remarkable optical properties. For example, the peak extinction wavelength (λmax), of their localized surface plasmon resonance (LSPR) spectrum is unexpectedly sensitive to nanoparticle size, shape, and local (~10‐30 nm) external dielectric environment, allowing development of a new class of nanoscale affinity sensors and biosensors.22,23 In particular, halide ions can trigger the oxidative etching of anisotropic noble nanoparticles, with iodide ions being the most efficient,24 and thus dramatically influence the optical properties of silver nanoprisms, making them an ideal candidate for the detection and quantification of iodide ions.25,26 In this paper we report a study on anisotropic L‐tyrosine capped silver nanoparticles (AgNPs) obtained by a new synthetic route that allows binding the un‐oxidized tyrosine on the nanoparticles surface. Even though amino acids have been used as capping agents for spherical nanoparticles, allowing the obtainment of very stable colloids with multiple properties,27,28,29,30 and short peptides have been used to replace the poly(vinylpyrrolidone) (PVP) on the surface of silver nanocubes,31 this is the first example, at the best of our knowledge, of amino acid capped anisotropic silver nanoparticles. These new nanoprims have been carefully characterized by a combined approach which implements the use of a solid state technique (TEM), as well as UV‐Vis spectroscopy and polarized and depolarized Dynamic Light Scattering (DLS and DDLS) in solution and their optical properties have been exploited for the quantitative determination of iodide ions in the presence of a large excess of chloride ([Cl‐]/[I‐] ≤ 1000). This selectivity is of crucial importance for practical applications. Chloride, in fact, is always present in tap waters, even if chloride levels in unpolluted waters are often below 10 mg/litre.32 The determination of iodide ions was also carried out in the presence of fluoride and bromide at the concentrations they are typically present in drinking water,32,33 resulting selective as well. This approach would be easily extendible to develop simple, rapid and sensitive assays for the determination of iodide concentration in tap waters, quickly assessing the necessity and the amount of iodide supplementation. Experimental part. 2.1. Materials. L‐Tyrosine (L‐Tyr), sodium fluoride (NaF), sodium chloride (NaCl), sodium bromide (NaBr) and potassium iodide (KI) were obtained as commercial reagents by Merck and were used as received. Silver nitrate (AgNO3), sodium borohydride (NaBH4), hydrogen peroxide (H2O2) and trisodium citrate (Na3Cit.2H2O) were purchased from Sigma Aldrich. 2.2. Synthesis of AgNPs. The L‐Tyrosine capped silver nanoparticles (L‐Tyr AgNPs) were synthesized by slightly modifying the procedures previously reported for citrate capped triangular AgNPs.34,35 To summarise, silver nitrate (0.1 M, 25 µL), L‐Tyrosine (2 mM, 2.06 ml), hydrogen peroxide (30 wt%, 60 µL) and sodium borohydride (100 µl 0.1 M) were successively added to 24.75 mL of water continuously magnetically stirred in the presence of air. The solution turned pale yellow and slightly cloudy. 0.022 gr of Na3Cit.2H2O and 100 µL of hydrogen peroxide were subsequently added and the solution became colourless. A further addition of NaBH4 gave rise to a solution colour change from colourless to red, green, and blue. 2.3. TEM analysis. Transmission electron microscopy was performed with a TEM JEOL JEM 2010 F, equipped with the Gatan imaging filter, operating at 200 keV. Samples for TEM were prepared by placing several drops of the colloidal dispersions of AgNPs onto a carbon‐coated copper micro‐grid (2 M STRUMENTI).
2.4. DLS measurements.
The Dynamic Light Scattering (DLS) measurements were carried out by a homemade apparatus as described elsewhere.36 The sample was lighted with a 633 nm He–Ne laser whose power ranged between 15 and 150 mW. The incident light was vertically polarized while the polarized (DLS) and depolarized (DDLS) scattering intensities were detected using a polarizer, positioned between the sample and the detector, enabling selective collection of either the vertically polarized scattering, VV, and the horizontally depolarized scattering, VH. The autocorrelation functions were measured at 90° corresponding to q=18 µm-1. The analysis of the fluctuations (due to the Brownian motion) of the scattered light was performed by the intensity auto‐correlation function (g2) defined as:
,
1 1 lim → 〈 〉
,
,
1
(1)
where I is the scattered light intensity, is the temporal average scattered light intensity, T is the acquisition time, q is the scattering vector defined as q = (4πn/λ)sin(θ/2), n is the refraction index of the solvent, λ is the light wavelength, and θ is the scattering angle. The field autocorrelation function g1 is computed from the g2 using the Siegert relation (g2=|g1|2 ).37 However, for spherical nanoparticles in Brownian motion the g1 function is a decreasing exponential with a relaxation rate Γ (Γ=1/ with τ = decay time), whereas for solutions containing anisotropic nanoparticles the g1 function shows several exponential decay components that were thus analysed either by cumulant or multi‐exponential analysis.38 From the vertically polarized scattering relaxation rate (ΓVV), it is possible to calculate either the translational diffusion coefficient (Dt) and the hydrodynamic radius (RH) using the following expressions:
ΓVV = Dtq2
6
(2) (3)
where k is the Boltzmann constant, T is the absolute temperature and is the water viscosity. The hydrodynamic radius is the radius of a hypothetic sphere that diffuses with the same Dt of the nanoparticle under examination. Analogously, the rotational diffusion coefficient can be determined by VH through the relation:
ΓVH = Dtq2 + 6Dr
(4)
The relations (2) and (4) allow evaluation of the translational and rotational diffusion coefficients, assuming a decoupling of rotational and translational diffusion mechanisms. We have already reported that this technique is useful to characterize plasmonic anisotropic nanoparticles, such as gold nano‐rods, nano‐clusters and silver nanoplatelets.36
2.5. UV–Vis spectroscopic measurements. UV–Vis spectra of the investigated systems were carried out on the undiluted silver sol at room temperature, using a diode‐array Agilent 8453 spectrophotometer in the 200–1100 nm wavelength range. The effect of F‐, Cl‐, Br‐ and I‐ ions on the UV–Vis spectrum of the triangular L‐Tyr capped AgNPs was investigated by adding appropriate amounts of each ion solutions (10‐5, 10‐4, 10‐3, 10‐2, 10‐1 or 1 M) to 2.0 mL of the silver sol, reaching a final concentration range of 2 x 10‐6‐ 7 x10‐5 for fluoride, 1 x 10‐6‐1 x 10‐3 M for chloride, 1 x 10‐6 ‐ 2.5 x 10‐5 for bromide and 1 x 10‐7 ‐ 5 x 10‐6 M for iodide, respectively. The determination of iodide was also carried out at a fixed concentration of the other halide ions to test the selectivity of the assay. Furthermore, in order to determine the I‐ concentration in a drinking water, a 0.200 ml sample of water was mixed with 1.800 ml of silver nanoparticles and was spiked with a iodide solution to obtain final I‐ concentrations ranging from 2.5 x 10‐7 M to 5 x 10‐6 M. UV‐Vis spectra were carried out on the resulting solutions.
Results and Discussion 3.1 Synthesis and characterization of L‐Tyr capped triangular AgNPs. 3.1.1 The synthesis of the triangular L‐Tyr capped nanoparticles was carried out by following the method reported in the reference 34, that overcomes the photoinduced39,40 and seed‐mediated41 syntheses. Yin et al. in fact, exploited the dynamic equilibrium between the reduction of silver ions by NaBH4 and the oxidative etching of metallic silver by H2O2. Furthermore, citrate, or other many di‐ and tricarboxylate compounds, (whose two nearest carboxylate groups are separated with two or three carbon atoms), if used in the appropriate ratio, play a fundamental role as capping agents, giving rise to well defined nanoplates. In this work, to obtain the triangular L‐Tyr AgNPs, L‐Tyr was added to the solution reaction in order to promote an additional capping. Tyrosine, in fact, is widely used as capping agent, either in the oxidized42 and in the not oxidized form,29 being able to bind the silver nanoparticles surface by the semiquinone group or the O‐ terminal group, respectively. 3.1.2 TEM The TEM images of the L‐Tyr AgNPs, reported in Figure 1a and 1b, clearly show the formation of few spherical nanoparticles and many nanoplate structures, most with a triangular (even if snipped) shape that strictly resembles a pick, i.e. a guitar plectrum. Stacks of particles, easily found on the film (Figure 1c), allowed an estimation of their thickness (~8 nm). The histogram in Figure 1d represents the Feret diameter distribution of the nanoprisms, that results, as average, 40 ± 5 nm. 3.1.3 UV‐Vis Spectroscopy. In Figure 2 the UV‐Vis spectra of the L‐Tyr AgNPs immediately after preparation, and after two weeks and three months of aging are reported. Even though the silver sol was stored in the dark to avoid photo‐induced effects on the shape and the geometrical characteristics of the nanoparticles, Figure 2 shows a temporal evolution in the recorded spectra, indicating evident aging occurring nonetheless. However, the spectra recorded after one and three months are similar showing that after around 30 days the sol is stable. The spectra show the typical characteristics reported in the literature for triangular nanoprisms,43,44 i.e. a strong dipole resonance at 636 nm, a weaker in‐plane quadrupole resonance at 433 nm, as well as a weak transversal out‐of‐ plane quadrupole band (334 nm). Indeed, the band for the dipole resonance was initially centred at λmax = 650 nm and gradually blue‐shifted to λmax = 636 nm, strongly indicating that the aging gave rise to a gradual snipping of the tips of the nanoprisms, as predicted by the theory of nanoparticle optical properties,43 the discrete dipole approximation (DDA) method45 and experimentally confirmed.46,47,48 In order to estimate the dimensions of the nanoparticles from the UV‐Vis spectra it should be considered that DDA calculations show that different particle thickness, in combination with modest differences in edge length, account for large differences in the UV‐Vis spectra of the nanoprisms colloids.49 Thus, from the UV‐Vis measurements an approximate value of 60 nm for the particles size was obtained,50 that agreed with the TEM results. Nevertheless, an accurate determination of the nanoparticles size requires a more in‐depth study of their shape, that is achievable by the DLS and DDLS measurements. 3.1.4. DLS. UV–Vis spectroscopy allows determination of several colloid properties, such as concentration, dimensions and shape of nanoparticles. However, this technique gives no dynamical information on the motion of the particles in solution. Furthermore, whereas standard DLS is widely used to determine the average NPs size, information about the shape can only be obtained by using a fast and accurate method such as depolarized dynamic light scattering.
In Figure 3 the normalized autocorrelation function g2,VV for L‐Tyr AgNPs is reported. The measured autocorrelation function displays an exponential trend that confirms the assumption of Brownian motion. The obtained value of Dt is 9.2x10‐8 cm2/s, corresponding to a value of dH= 51 ± 4 nm. This is in agreement with the dimensions determined by TEM and UV‐Vis spectroscopy. In the same Figure the normalized autocorrelation function g2,VH for L‐Tyr AgNPs, which shows a decreasing exponential with a relaxation rate VH =49487 s‐1, is also shown. Using the equation (4), we obtained a Dr value of 8247 s‐1. The high value of the depolarized scattered signal confirms the anisotropic shape of the nanoparticles. In fact, depolarized scattering intensity is proportional to the “optical anisotropy”, i.e. the difference between polarizability parallel and perpendicular to the principal symmetry axis), whereas in the case of spherically symmetrical structures the rotation around an axis does not involve optical anisotropy and therefore does not give a contribution to the depolarized DLS signal. Anisotropically shaped nanoparticles with two or three different orthogonal dimensions could be related to ellipsoidal shapes, either nanooblates and nanoprolates. In particular, oblate shapes, including nanoplatelets and nanodisks, are characterized by one short “thickness” and two more or less similar, larger “widths”. The two distinctly different dimensions give rise to different spectroscopic characteristics which strongly depend on the aspect ratio of the nanoparticles.51 Using the classical model of Perrin,52,53 as reported in a previous paper,54 it was possible to calculate the values of the major and minor axes (a and b) of the ellipsoid, by using the two independent measured values of Dt and Dr. The obtained values are a = 14 nm and b= 70 nm, with an aspect ratio =b/a of 5. Thus, the appropriate use of dynamic light scattering methods allows an accurate determination of the shape and the size of nanoparticles, showing several advantages with respect to TEM. In fact, whereas TEM gives information on the systems under investigation after the removal of the solvent, the DLS and DDLS allow a careful characterization of the same systems in solution. Furthermore, these techniques are less expensive and faster than microscopic techniques. 3.2. Halide ion‐sensory properties of pick silver nanoparticles. The UV‐Vis spectra of the pick silver nanoparticles at different concentrations of chloride ions are reported in Figure 4. The Figure clearly shows that in 0‐1000 µM range the shape of SPR peak did not change and the differences in the absorbance values were only due to the dilution effects (within the experimental error). On the other hand, increasing the Cl‐ ions concentration from 0.01 M to 0.10 M gave rise to a progressive and dramatic blue‐shift of the absorption peak that indicated, at first, a pronounced snipping of the tips of the pick nanoparticles until, in the end, they became discoidal, as previously reported.55 However, in the paper of W. Xu et al., the chloride ions readily attacked the silver atoms at the vertex area that were selectively etched, starting just from a concentration of 1000 µM. In our case, instead, rounding of the nanoprisms was only observed at a concentration of chloride ions of nearly 0.01 M. This different threshold in the chloride concentration was probably due to the better performance of tyrosine as capping agent, that limited the “sculpturing effect” of chloride ions in shape transformation from triangular to discal silver nanoplates55 Hence, the UV‐Vis spectrum of the nanoparticles was not very sensitive to the chloride ions, being capable of their detection only at concentrations above 1000 µM. The addition of iodide ions gave rise to a quite different trend. In fact, the facet‐selecting etching of the nanoprisms was driven by the chemical characteristics of the etching agents. As it is well known, halide ions are precipitant agents for the silver ions and, between the resulting insoluble compounds, AgI is much more insoluble than AgCl. Thus, iodide ions can react with the active silver atoms, dissociating them from the nanostructure56 and subtracting them from the equilibrium much better than the chloride ones. Furthermore, iodide is the most efficient halide ion for the oxidative
etching of anisotropic noble nanoparticles24. However, at our experimental conditions the basic pH of the solution probably rules out the oxidation of iodide in iodine, as reported for bromide57 and iodide58 in weak acid solutions. The data reported in Figure 5a show at first (until to 2.9 µM) a decrease in the absorption of the SPR peak, that is undoubtedly not ascribable to the dilution, but instead to a partial snipping of the tips of the AgNPs. The population of the nanoparticles initially absorbing at 636 nm decreased, whereas the corresponding signal of the resulting “rounded” plates was too low to be detected. Above 3 µM the in‐plane dipole plasmon resonance peak at 636 nm blue‐shifted and its absorbance dramatically decreased, indicating a more important etching and thus an additional snipping of the tips of the triangular nanoparticles by the iodide ions. This gave rise to the formation of nanoplates55 instead of spherical particles, as indicated by the absorption of the SPR peak centred at 504 nm.46 Above 5 µM, no significant changes in the UV‐Vis spectra were detected; this could indicate that the snipping process is complete and cannot go further and that all the pick nanoparticles have been transformed into oblate nanoplates. This trend is opposite to the results reported by Yang et al.26 In this paper, in fact, raising the concentration of iodide ions transformed the nanoprims into spherical particles, as testified by the disappearance of the absorption bands due to the quadrupole oscillations, still present instead in the L‐Tyr capped AgNPs. This opposite mechanism is probably due to the improved performance of tyrosine as capping agent; in fact, this amino acid prevents the direct action of iodide on the nanoparticles surface that could induce their fusion into spherical symmetry aggregates, favouring instead the etching action and the resulting snipping of the tips. The A636‐A317 of the silver nanoparticles at different concentrations of iodide ions are plotted in Figure 5b. At low concentrations, the absorbance values decreased linearly with an increase in the concentration of iodide up to 2.9 μM. However, when the I‐ concentrations were above 3 μM, a straight line with a more negative slope was observed. The change of the slope could be ascribed to the fact that the etching process critically depended on the concentration of iodide ions that induced a different morphology of the nanostructures.59,60 In fact, whereas at low iodide ion concentrations the nanoparticles still had a pick shape, and thus the tips, even if rounded, were still present, representing a preferential reaction site, above 3 μM, the snipping process was almost complete and the I‐ species interacted in the same way either on the edges of the particles and on their flat surfaces (see Figure 6), lowering the thickness of the nanoplates and consequently their symmetry and ultimately giving rise to an increase of the peak due to the oscillation of the in‐plane quadrupole. Furthermore, as reported in Figure 7, even in the presence of a concentration 1000 µM of Cl‐, the UV‐Vis spectra of the pick‐shaped silver nanoparticles showed the same trend as a result of the addition of increasing concentrations of iodide ions. Up to a concentration of iodide ions of 2.9 µM, in fact, the absorbance of the in‐plane dipole plasmon resonance peak at 636 nm decreased, whereas above 3 µM this peak blue‐shifted and its absorbance dramatically decreased (Figure 7a). The plot of the A636‐A317 as a function of the concentration of the iodide ions (Figure 7b) showed a break point also in this case, even if the slopes of the straight lines were minor than those reported in Figure 5 that are referred to the measurements carried out in the absence of chloride ions. Obviously, the presence of chloride ions adsorbed on the surface of the nanoparticles renders them less prone to the etching of iodide ions. We also tested the effect of fluoride and bromide ions at their typical upper limits in drinking water (i. e. 70 and 24 μM, respectively) on the spectral characteristics of the pick‐shaped silver nanoparticles and the results are shown in Figure 8a and 8b, respectively. Unlike what was found for the chloride ions, either fluoride and bromide, already from a concentration of few micromolar, gave
rise to a decrease of the SPR absorbance that was clearly not ascribable solely to the dilution effects. The effect is more pronounced for bromide ions that above 10 μM also gave rise to a blue shift of the plasmon resonance peak. The changes of the surface plasmon peak of the pick‐shaped silver nanoparticles at a concentration of 5 μM of the four halides, summarized in Figures 8c‐e, could be easily fathomable, as the effects on the SPR peak increased as the solubility products of the silver halides decreased, with the exception of the effect of the chloride ions, that could, at first sight, appear odd. Nevertheless, it should be considered that silver ions form complex species with Cl‐ ions only, and that these species could compete with the formation of insoluble silver halides. In fact, in order to obtain the etching of anisotropic L‐Tyr silver nanoparticles it was necessary to reach a concentration of chloride ions major than 1000 μM. In drinking water the average concentration of F‐ and Br‐ ions rarely exceeds 0.1 and 0.3 ppm, respectively32,33 and thus their interference with the determination of iodide was evaluated at this concentration. In both cases the plots of the A636‐A317 as a function of the concentration of the iodide ions (Figure 8g and 8h) show a break point at a value slightly lower than those observed in the measurements carried out in the absence of other halide ions (Figure 5), even if the slopes of the straight lines are lower. Obviously, as previously observed for the measurements carried out in the presence of chloride ions, fluoride and bromide ions adsorbed on the surface of the nanoparticles renders them less prone to the etching of iodide ions. In order to evaluate the robustness of the analytical method for the determination of iodide ions in drinking waters proposed here, a sample of an oligomineral water distributed in Italy was analysed. To achieve this, several experiments were carried out by mixing different volumes of the silver colloid and the drinking water. The optimal ratio of volumes was found to be 1.800/0.200 ml of silver sol/drinking water. In fact, at this ratio, it was possible to evaluate the effect of ionic strength of the water on the nanoparticles without diluting the solution significantly. The UV‐Vis spectra of the resulting solution spiked with different concentrations of iodide ions are reported in Figure 9. The data showed a trend that is very similar to that concerning the addition of iodide to the silver colloid alone (Figure 5a). In fact, it was possible to observe at first (until to 2.5 µM) a decrease in the absorption of the SPR peak, not ascribable to the dilution alone, but instead to a partial snipping of the tips of the AgNPs. Above 2.5 µM the in‐plane dipole plasmon resonance peak at 636 nm blue‐ shifted and its absorbance dramatically decreased, indicating a more important etching. Above 4 µM, no significant changes in the UV‐Vis spectra were detected; this could indicate that the snipping process was complete and could not go further, and that all the pick‐shaped nanoparticles were transformed into oblate nanoplates. The only difference between the two trends was the values at which there is a variation of the slope (3.0 µM for pure sol and 2.5 µM for the drinking water, respectively), and at which no significant changes in the UV‐Vis spectra were detected (5.0 µM for pure sol and 3.8 µM for the drinking water, respectively). Probably, even though the analysed water was a light mineral one (fixed residue 22 mg/litre), the dissolved electrolytes gave rise to a small, but detectable, etching effect that made the variation of slopes and the saturation effect appear at concentrations lower than those observed for the colloid alone. Conclusions. New L‐Tyr capped silver anisotropic nanoparticles were synthetized and accurately characterized. The presence of tyrosine as a capping agent gave rise to the formation of well stable pick‐shaped particles. The use of different techniques (TEM, UV–Vis spectroscopy and DLS) allowed us not only to fully characterize novel L‐Tyr capped AgNPs, that showed a pronounced anisotropy, but also to smartly exploit them for the quantitative determination of I‐ ions in solution, even in the presence of a large excess of chloride ions. Thus, pick‐shaped nanoparticles could be used to develop simple,
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Figure(s)
b)
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d) Frequency
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Feret diameter (nm)
Figure 1. (a-c) Transmission electron microscope images of L-Tyr capped AgNPs (c) Stacks of particles; (d) Histogram of the Feret diameters of the nanoprisms in the sample (average size: 40.0 ± 5 nm).
Figure(s)
1.0
L-Tyr AgNPs after 3 days L-Tyr AgNps after 17 days L-Tyr AgNPs after 3 months
0.8
A
0.6 0.4 0.2 0.0
400
600
800
1000
(nm)
Figure 2. UV–Vis spectra recorded for anisotropic L-Tyr capped AgNPs.
Figure(s)
G2 (Arb. Units)
1
0.1
G2,VV G2,VH 0.01
0.0
5.0x10
-5
1.0x10
-4
Time (s)
1.5x10
-4
Figure(s)
0.6 0
0.4
[Cl-] 1000 μM
0.0100 M [Cl-]
A
0.1000 M
0.2
0.0 400
600
800
1000
(nm) Figure 4. UV-Vis spectra of anisotropic L-Tyr capped AgNPs at different concentrations of chloride ions.
Figure(s)
1.0
0
a)
A
[I-] 5 μM
0.5
0.0 400
600
800
1000
(nm) 0.8
A636-A317
0.6
Y = 0.848 -92255x R2 = 0.99354
b)
0.4 0.2 0.0 0.00
Y = 1.34 -258121x R2 = 0.99742 -6
-6
1.00 2.00x10 2.00 3.00 4.00x10 4.00 [I ] M
Figure 5. UV-Vis a) Spectra of anisotropic L-Tyr AgNPs as a function of various concentrations of I- ions. (b) Plot of A636-A317 vs the concentration of the iodide ions.
1.0
1.0
0.8
0.8
0.8
0.6
0.6
0.6
0.4 0.2 0.0
A
1.0
A
A
Figure(s)
0.4 0.2
400
600
800
(nm)
1000
0.0
0.4 0.2
400
600
800
(nm)
1000
0.0
400
600
800
(nm)
Figure 6. Schematic representation of the iodide ions pick particles interaction.
1000
Figure(s)
1.0 0.8
0
a)
[I-]
A
0.6
10 μM
0.4 0.2 0.0
400
600
800
1000
(nm)
A636-A317
0.8 0.6
b)
0.4
Y = 0.788 -63610x R2 = 0.98996
Y = 0.892 -107032x R2 = 0.98776
0.2 0.0 -6 -6 0.00 1.00 2.00x10 2.00 - 3.004.00x10 4.00 5.00 [I ] M Figure 7. UV-Vis a) Spectra of anisotropic L-Tyr AgNPs as a function of various concentrations of I- ions in presence of 1000 µM [Cl-]. (b) Plot of A636-A317 vs the concentration of the iodide ions.
Figure(s) 1.2
1.2
1.0
a)
0.8
1.0
[F-]
0.6
70 μM
0.4
0.2
0.2 400
600
800
1000
24 μM
0.0 200
400
(nm)
c)
1.0
0
[F-]
0.6
5 μM
0.4
0 [Cl-]
0.6
5 μM
0.4
0.2
0.2
0.0 200
400
600
800
0.0 200
1000
400
(nm)
600
800
1000
(nm)
1.2
1.2
0
e)
1.0
0.6
0.6
0.4
0.4
0.2
0.2
0.0 200
400
600
800
A636 - A317
0.6
0.4
[F-] = 5 μM Y = 0.99 -200000x R2 = 0.99699
0.0 0.00
1.00
2.00 3.00 [I ] M
400
4.00
5.00
600
800
1000
(nm)
0.8
Y = 0.795 -130000x R2 = 0.99172
g)
5 μM
0.0 200
1000
(nm)
0.8
[I-]
0.8
A
5 μM
0
f)
1.0
[Br-]
0.8
A
1000
d)
0.8
A
A
0.8
A636 - A317
800
1.2
1.0
0.2
600
(nm)
1.2
0.6
[Br-]
0.6
0.4
0.0 200
0
b)
0.8
A
A
0
Y = 0.756 -99000x R2 = 0.99678
h)
0.4 0.2 0.0 0.00
[Br-] = 4 μM Y = 1.00 -196000x R2 = 0.99733 1.00
2.00 3.00 [I ] M
4.00
Figure 8. a)-f) UV-Vis spectra of anisotropic L-Tyr capped AgNPs at different concentrations of halide ions; plot of A636A317 vs the concentration of the iodide ions in the presence of g) fluoride and h) bromide ions.
5.00
Figure(s)
1.2
0.8
0
1.0
[I-]
0.6
5 μM
0.4 0.2 0.0 200
400
600
(nm)
800
1000
A636 - A317
A
0.8
0.6 0.4 0.2 0.0 0.00 0.0
1.00 -6 2.0x10 2.00 -6 3.0x10 3.00 -6 4.0x10 4.00 -6 1.0x10 -
[I ] M
Figure 9. a) UV-Vis spectra of anisotropic L-Tyr capped AgNPs mixed with a sample of drinking water and spiked with different concentrations of iodide ions; b) corresponding plot of A636-A317 vs the concentration of the iodide ions.