A facile photochemical route for the synthesis of ...

Journal of Photochemistry and Photobiology A: Chemistry 270 (2013) 1–6

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A facile photochemical route for the synthesis of triangular Ag nanoplates and colorimetric sensing of H2 O2 Raj Kumar Bera, C. Retna Raj ∗ Department of Chemistry, Indian Institute of Technology, Kharagpur 721308, India

a r t i c l e

i n f o

Article history: Received 4 May 2013 Received in revised form 25 June 2013 Accepted 12 July 2013 Available online 23 July 2013 Keywords: Photo-induced electron transfer Ag nanoplate Colorimetric sensing Oxidative etching Hydrogen peroxide

a b s t r a c t We describe a facile photochemical route for the synthesis of triangular Ag nanoplates using NADH model compound, N-benzyl-1,4-dihydronicotinamide (BNAH) and the shape-dependent colorimetric sensing of H2 O2 . Synthesis of Ag nanoplates involves the irradiation of an aqueous mixture of Ag(I) and BNAH in the presence of trisodium citrate with sunlight for 30 min. The photoexcited BNAH reduces Ag(I) to Ag(0) and quasi-spherical nanoparticles were obtained at the initial stage of the reaction. The spherical nanoparticles undergo shape transformation by light and yield nanoparticles of different shapes. The spherical nanoparticles first undergo light-induced transformation to hexagonal nanoplates (∼50 nm) and then to triangular nanoplates (40 nm) in 30 min of irradiation. The triangular nanoplates exhibit two main bands corresponding to the quadrupole and dipole in-plane plasmon resonance at 407 and 620 nm, respectively, along with a shoulder band ∼335 nm corresponding to out-of-plane quadrupole resonance. The Ag nanostructure of different shapes has been used for the colorimetric sensing of H2 O2 . The thermodynamically favorable oxidative etching of triangular Ag nanoplates by H2 O2 turns the initial green color of Ag nanostructures to gray. In the presence of H2 O2 , the surface plasmon band at 407 nm completely vanishes and the absorbance of the band at 620 nm significantly decreases. The oxidative etching transforms the triangular nanoplates into nanodisc with an average size of 15 nm in 1 min. The shape transformationinduced visible color change was used for the quantification of H2 O2 . Interestingly, the oxidative etching reaction depends on the shape of the nanoparticles. The shape of the Ag nanoparticles actually controls the redox reaction. The triangular Ag nanoplates are highly sensitive toward H2 O2 and it could detect H2 O2 at sub-micro molar level without any interference from coexisting other analytes. The high reactivity of the triangular nanoplates can be ascribed to the existence of coordinatively unsaturated reactive atoms at the corners of the triangular nanostructure. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The synthesis of Ag nanoparticles received significant attention owing to the fascinating optical properties. The interesting properties have been exploited for sensing, drug delivery, imaging, etc. [1]. Among the noble metals, Ag nanoparticles exhibit the highest efficiency of plasmon excitation [2]. They interact more efficiently with visible light than any organic/inorganic chromophore [3]. Ag nanoparticle can capture more light than its physical size limit, as the absorption or scattering cross section exceeds the geometric cross section of the particle [3]. The optical property of Ag nanoparticles strongly depends on the size, shape, dielectric constant of the surrounding medium, and interparticle distance. Among the various shapes, triangular Ag nanoprisms/nanoplates are very promising for various applications [4]. The surface plasmon band of Ag nanoplates can be easily tuned

∗ Corresponding author. Tel.: +91 3222283348; fax: +91 3222282252. E-mail address: [email protected] (C.R. Raj). 1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2013.07.005

by controlling the aspect ratio [5]. In the past, a variety of synthetic methods have been developed for the synthesis of library of Ag nanostructures with different shapes and size in aqueous and non-aqueous media [4–11]. The chemical and photochemical routes with shape regulating reagents have been employed in the past for the synthesis of triangular nanoprisms/nanoplates [5,11]. The chemical routes involve the use of multiple reagents such as citrate, H2 O2 , polyvinylpyrolidone, etc. In the photochemical synthesis, the Ag nanoplates were obtained by tuning the wavelength of the light source [10]. The photochemical conversion of spherical Ag nanoparticles to nanoprism/nanoplate was achieved using various stabilizing agents [9]. One of the primary requirements in the photochemical conversion of spherical Ag nanoparticles to nanoplate is the overlap of the incident light with surface plasmon of the nanoparticles [5]. In most of the cases, the Ag nanoparticles undergo shape transformation first to triangular nanoplates and then to hexagonal nanoplates. To the best of our knowledge, only one report describes the photo-induced shape transformation of hexagonal to triangular nanoplates [12].

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R.K. Bera, C.R. Raj / Journal of Photochemistry and Photobiology A: Chemistry 270 (2013) 1–6

Fig. 1. TEM images of Ag nanostructure obtained at (a) 5 min, (b) 10 min, (c) 20 min, and (d) 30 min of the reaction. The HRTEM and SEAD pattern for the triangular Ag nanoplates are shown in (e) and (f). Selected hexagonal and triangular plates are shown in (g) and (h).

H2 O2 is an oxygen metabolite and has dual role in the living system. It has been considered as a “necessary evil” due to the fact that it involves in the redox signaling for the normal function and it is an unwanted “killing agent” [13]. The abnormal generation of H2 O2 induces oxidative stress and is associated with aging and cancer. Moreover, H2 O2 is traditionally used as a food additive to control the growth of microorganism and bleaching. According to the “Code of Federal Regulations” H2 O2 was usually accepted as safe material for the use as a bleaching agent in agreement with good manufacturing practice [14]. In Japan, H2 O2 was used as a food additive for sterilizing and bleaching purposes until Feb 1980 [15]. Later, the Standards and Specifications were partially modified [15] as the drinking water administered with at 0.1–0.4% of H2 O2 was found to induce cancer in the duodenum of mouse [16]. The sensing of H2 O2 has also received enormous interest in the development of biosensors [17–19]; the working function of oxidase-based biosensors involves the sensing of enzymatically generated H2 O2 [17–19]. Several enzymatic [20] and non-enzymatic [21] analytical methods have been used for the detection and quantification of H2 O2 [20–23]. Willner’s group successfully exploited the fascinating optical properties of metal and metal oxide nanoparticles in the development of oxidase enzyme-based biosensing protocols [18,19]. Recently, Xia’s group demonstrated the tailoring of the optical property of Au–Ag alloy nanoboxes and the measurement of H2 O2 [24]. Fan’s group demonstrated that the in situ generated H2 O2 by Au nanoparticle catalyzed glucose oxidation induces the seed mediated growth of Au nanoparticles [25]. Although various methods have been proposed, the development of a simple cost-effective analytical protocol for the selective, reliable sensing of H2 O2 is still a challenging task. Our group is interested in the development of optical and electrochemical sensing methodologies for various analytes using nanoscale functional materials [26,27]. In continuation of our earlier works [26] in the development of photochemical route using visible light, herein, we describe a new photo-assisted route for the synthesis of triangular Ag nanoplates in aqueous medium using NADH model compound BNAH in presence of trisodium citrate as a stabilizer and the shape-dependent optical sensing of H2 O2 at sub-micromolar level based on the shape transformation-induced visible color change. 2. Experimental 2.1. Materials AgNO3 , H2 O2 , and trisodium citrate were purchased from Merck and BNAH was purchased from Tokyo Chemical Industry Co., Ltd. All

other chemicals used in this investigation were of analytical grade and used without further purification. 2.2. Instrumentation Electronic absorption spectra were taken using CARY 5000 UV–visible-NIR spectrophotometer. TEM images were acquired using JEOL JEM-2010 microscopes with an operating voltage of 200 kV. XRD patterns of the samples were collected using Panalytical X’pert PRO XRD unit with nickel-filtered Cu K␣ radiation ˚ Electrochemical experiments were performed with ( = 1.54 A). CHI643B electrochemical analyzer (CHI) using a two-compartment, three-electrode electrochemical cell. A glassy carbon (GC) working electrode, Ag/AgCl (3 M KCl) reference electrode, and Pt wire auxiliary electrodes were used in the electrochemical studies. 2.3. Synthesis of Ag nanostructures In a typical single step synthetic procedure, aqueous solution of 90 ␮L of AgNO3 (20 mM), 2.86 mL of BNAH (0.32 mM), and 50 ␮L of trisodium citrate (40 mM) were mixed and irradiated using a 125-W sunlamp with wavelengths >350 nm for 30 min. Polyhedral shape Ag nanoparticles were synthesized according to our previous report [26]. 2.4. Optical sensing of H2 O2 For the optical sensing of H2 O2 , the as-synthesized colloidal Ag nanoparticles were quantitatively diluted as the absorbance was significantly high. The typical procedure for the optical sensing involves the addition of aliquots of H2 O2 solution to different vials containing 3 mL Ag nanoparticles. The spectral measurements were performed 1 min after the addition of H2 O2 . 3. Result and discussion 3.1. Characterization and growth mechanism Fig. 1 displays the TEM images of Ag nanostructures acquired at different time interval during the reaction. The size and shape of the nanostructures depend on the irradiation time. At the initial stage (5 min), spherical and quasi-spherical Ag nanostructures with an average size of 10 nm were obtained whereas distorted hexagonal nanoplates with an average size of 50 nm were obtained at 10 min of irradiation. The edge lengths of hexagonal nanoplates are not


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Fig. 2. Time-dependent spectral profile illustrating the growth of different Ag nanostructures. [AgNO3 ]: 0.6 mM; [BNAH]: 0.3 mM; [Na-citrate]: 0.66 mM. Inset is the digital photographs of the corresponding colloidal nanoparticles.

identical and it varies between 15 and 20 nm. Nanoparticles obtained at 20 min of irradiation have distorted hexagonal (∼50 nm), spherical (∼10 nm), and triangular (∼40 nm) shapes. The sample after 30 min of the irradiation shows triangular Ag nanoplates (∼40 nm) along with few quasi-spherical nanoparticles (∼5 nm). The triangular plates have an edge length of 40 nm. The HRTEM image (Fig. 1) of the triangular Ag nanoplate shows ˚ corresponding to the interplanar spacthe fringe spacing of 2.38 A, ing between (1 1 1) planes. The selected-area electron diffraction pattern obtained from triangular Ag nanoplate was indexed to the respective planes of a face-centered cubic lattice. The XRD profile of the Ag nanostructure obtained at different time intervals shows the presence of predominant (1 1 1) faces (Fig. S1 in the Supporting Information). The mechanism involved in the reduction of Ag(I) and the growth of the nanoplates were understood by time-dependent spectral measurements. The initial colorless reaction mixture turned to yellow immediately after irradiation and then to orange within 10 min. The orange color became light green after 20 min and deep green color appeared after 30 min of the reaction (Fig. 2, inset). The time-dependent spectral profile obtained during the reaction is shown in Fig. 2. Initially, a strong absorption band corresponding to the initial formation of spherical/quasi-spherical Ag nanoparticles was observed ∼397 nm. Development of an additional band ∼490 nm at the expense of the initial band (∼397 nm) was noticed during the course of the reaction (10 min). The bands at 397 and 490 nm shifted to higher wavelength side and a new band ∼620 nm was observed at the expense of the other two bands during the subsequent irradiation for another 10 min. The absorbance of new band continues to increase with time and attained the saturation at 30 min of irradiation. The two main bands at 407 (quadrupole resonance) and 620 nm (dipole in-plane resonance) and the small shoulder band ∼335 nm (out-of-plane quadrupole resonance) are attributed to the growth of triangular nanoplates. No further change in the spectral profile was observed after 30 min of the reaction. The time-dependent spectral feature indicates the shape transformation of the nanoparticles and is in agreement with the TEM measurements. The reduction of Ag(I) by BNAH in the absence of trisodium citrate does not proceed in dark. However, BNAH could reduce Ag(I) in the absence of trisodium citrate in sunlight and yields quasi-spherical nanoparticles (Fig. S2 in the Supporting Information), indicating that light controls the reduction of Ag(I) and the growth of nanoparticles. In the absence of citrate, BNAH could not favor the growth of triangular Ag nanoplates. Both citrate and BNAH are required for the growth of triangular nanoplate. Scheme 1 illustrates the probable mechanism involved in the photo-induced electron transfer from BNAH to Ag(I) and the growth of anisotropic

Scheme 1. Scheme illustrating the photo-induced electron transfer between BNAH and Ag(I) and the growth of triangular Ag nanoplates.

Ag nanostructures. In the ground state, BNAH is not capable of reducing Ag(I), owing to the high redox potential of BNAH+• /BNAH couple (0.81 V) [28] with respect to Ag(I)/Ag(0) couple (0.8 V). The UV–vis spectrum of BNAH shows a strong absorption band in the UVA region with max of 360 nm (Fig. S3 in the Supporting Information). The BNAH* generated during irradiation can be an excellent electron donor [29]. In our case, the BNAH* efficiently reduces Ag(I) to Ag(0). The photo-induced one electron reduction of Ag(I) produces Ag(0) and BNAH+• . This cation radical subsequently undergoes deprotonation to yield the radical BNA• [28]. The redox potential of the BNA+ /BNA• (−0.84 V) [28] is significantly lower than that of Ag(I)/Ag(0) couple and BNA• is thermodynamically capable of reducing Ag(I) to Ag(0) according to Scheme 1. The photochemically produced Ag(0) nucleates to yield spherical Ag nanoparticles at the initial stage and subsequently undergo light-induced fusion to form distorted hexagonal nanoplate with six corners. The hexagonal nanoplates undergo further transformation to yield triangular nanoplates. The alternate edges of hcp layer are composed of (1 1 1) and (1 0 0) faces and it has been proposed earlier that the (1 0 0) faces are less stable with respect to the (1 1 1) faces [8]. Moreover, the hcp structure of Ag is less stable than the fcc structure, implying that the edges where the exposed hcp structure are less stable [8]. In such case, it is proposed that the atoms at the less stable faces (1 0 0) can undergo migration, diffusion or dissolution in presence of light to produce triangular nanoplate. Though the exact mechanism is not understood, the shape transformation of hexagonal to triangular nanoplate may occur through any of the following three probable pathways: (i) upon irradiation, the high energy surface atoms at the selective corners of the hexagonal nanoplate migrate to other places [30], (ii) the high energetic atoms at the selective corners simply diffuse toward the planner surface [30], or (iii) the high energetic surface atoms dissolve first and then reduce back by trisodium citrate in presence of light and can be deposited over the two faces of the planner structure [30]. In all three possible pathways, the thickness of triangular nanoplate would eventually increase and size of the nanoparticles would decrease. In our case, all these possibilities are ruled out as the thickness remains almost same (∼4 nm) in both hexagonal and triangular nanoplates (Fig. S4 in the Supporting Information). It is considered that the atoms at the corner of the hexagonal plate undergo dissolution during the reaction and the

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Fig. 4. TEM image of the Ag nanoparticles obtained after the addition of H2 O2 (12 ␮M). Inset is the HRTEM image of the selected Ag nanodisc.

Fig. 3. (a) Optical spectra illustrating the colorimetric sensing of H2 O2 using triangular Ag nanoplate. (b) Digital photograph showing the visible color change in the presence of H2 O2 (12 ␮M). (c) Calibration plot corresponding to the colorimetric titration.

in situ generated Ag(I) are reduced back to Ag(0) and form spherical nanoparticles. The Ag(0) prefer to nucleate to form spherical/quasispherical nanoparticles rather than to occupy the surface of the parent hexagonal nanoplates, possibly due to the capping of the base (1 1 1) plane by citrate ions [31]. The TEM image confirms the existence of spherical Ag nanoparticles. It is worth pointing out here that the shape and size of the nanoparticles can be controlled by tuning the structure of the reducing agent. For instance, recently, we have demonstrated the photochemical synthesis of polyhedral Ag nanoparticles using NADH as reducing agent [26]. The adenine dinucleotide moiety of NADH actually stabilized the nanoparticles and favored the growth of polyhedral shape. In the present case, although BNAH has the same redox moiety as NADH, it does not have the adenine dinucleotide moiety. It is obvious that the difference in the structure of the reducing agent is one of the key factors in regulating the shape of the nanoparticles. 3.2. Optical sensing of H2 O2 The redox potential of the H2 O2 /H2 O couple (1.763 V in acidic medium) or H2 O2 /OH− couple (0.867 V in basic medium) [24] is higher than that of the Ag(I)/Ag couple (0.8 V) and H2 O2 can efficiently oxidize Ag nanoparticles in both acidic and basic medium. In our case, the pH of different colloidal Ag nanoparticles was found to be in the range of 5–6 and they can undergo spontaneous redox reaction with H2 O2 . Such redox reaction can be effectively used to control the shape and size of the nanoparticles. Because the optical properties of Ag nanoparticles strongly depend on the shape and size, the oxidative etching reaction can induce a visible color change and hence H2 O2 can be quantitatively measured. Fig. 3a

displays the optical spectra of triangular Ag nanoplates in the presence of different concentration of H2 O2 . A gradual decrease in the absorbance of both the bands at 407 and 620 nm and a blue shift in the band at 620 nm were noticed while increasing the concentration of H2 O2 . The decrease in the absorbance of the bands has been used to quantify the concentration of H2 O2 . At high concentration (12 ␮M), the band at 407 nm completely vanished and only one band at 592 nm was observed. The deep green color of the colloidal Ag nanostructure turned to gray immediately after the addition of H2 O2 . The disappearance of the quadrupole plasmon resonance band (407 nm) and decrease in the absorbance of dipole in-plane plasmon resonance band (620 nm) associated with the blue shift upon the addition of H2 O2 points out the conversion of triangular shape nanoparticles to nanoparticles of other shapes by oxidative etching. The TEM measurements reveal the transformation of triangular nanoplate to disc like nanoparticles (Fig. 4 and Supporting Information S5). The redox reaction between Ag nanostructures and H2 O2 convert the triangular shape to nanoparticles of disc shape with an average size of 15 nm. It is worth pointing out that the band at 335 nm corresponding to the out-of-plane quadrupole resonance does not change in the presence of H2 O2 . The calibration plot was made with the absorbance at 620 nm and is linear up to the concentration of 8 ␮M (Fig. 3b). The detection limit (3) was found to be 0.04 ␮M. It should be mentioned here that the decrease in the absorbance ∼407 nm could also be ascribed to the participation of coexisting spherical nanoparticles in the oxidative etching reaction with H2 O2 . The spherical nanoparticle can undergo oxidative etching, as evidenced from the decrease in the absorbance (vide infra, Fig. S6). However, the observed red shift in the quadrupole band (∼400 nm) along with decrease in absorbance (Fig. 4) reveals the effective participation of triangular nanoplate in the oxidative etching reaction by H2 O2 . The oxidative etching of spherical nanoparticles can only cause the decrease in the absorbance (vide infra, Fig. S6). It is interesting to note that the shape of the nanoparticles can control the reactivity. It is well known that the nanoparticles with edge and corner have significantly high activity with respect to the other nanoparticles. Because shape of the nanoparticles regulates the optical properties, it is expected that the sensitivity of colorimetric sensing approach can be controlled by tuning the shape of the particles. The reactivity of Ag nanostructures of spherical, polyhedral, and triangular shapes toward H2 O2 was examined. The spectral feature of these Ag nanoparticles in the presence and absence of H2 O2 are summarized in Table 1 and Fig. S6. The


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Table 1 Table illustrating the shape-dependent optical sensing of H2 O2 and the oxidation of potential of the Ag nanoparticles. Shape

Triangular Polyhedral Spherical a

% of decrease in absorbance

Oxidation potential

Quadrupole band

Dipole in-plane band

V vs Ag/AgCl)

[H2 O2 ]L a

[H2 O2 ]H a

[H2 O2 ]L

[H2 O2 ]H

56 2 11

95 5 55

41 3 –

87 5 –

0.38 0.43 0.4

[H2 O2 ]L = 2 ␮M and [H2 O2 ]H = 12 ␮M.

activity of Ag nanoparticles toward H2 O2 increases in the order of polyhedral < spherical < triangular nanoplate. The triangular Ag nanoplates are highly sensitive toward H2 O2 . A 41–56% decrease in the absorbance of the bands at 620 and 407 nm was observed at low concentration of H2 O2 (2 ␮M). However, 87–95% decrease was observed at high concentration of H2 O2 (12 ␮M). The polyhedral shape Ag nanoparticle shows very poor response even at high concentration of H2 O2 (12 ␮M). The polyhedral nanoparticles do not undergo facile oxidative etching. On the other hand, the spherical Ag nanoparticle shows ∼11% decrease in the absorbance at 2 ␮M of H2 O2 . The high activity of triangular nanoplates toward H2 O2 probably originates from the defect sites at the vertices. The nanoparticles with corner site have relatively higher activity for oxidative etching, because of the presence of inherent defects [32]. The redox reaction preferentially occurs at the vertices of the triangular nanostructures because of the high surface energy at the vertices [33]. Hong et al. demonstrated that the vertex atoms are more labile than the surface atoms depending on the chemical change of the medium [34]. In order to rationalize the shape-dependent activity of Ag nanoparticles, electrochemical experiments using linear sweep voltammetry was performed for all the nanoparticles. As shown in Table 1 and Fig. S7, the oxidation potential of triangular plate is less positive (0.38 V) than those of other nanostructures. This high reactivity of triangular plate toward electrochemical oxidation may due to the high reactive atom at the corner with low coordination number [34]. The oxidation potential of the spherical Ag nanoparticles is less positive with respect to the polyhedral Ag nanostructure. Ming et al. demonstrated that shape of the nanoparticles controls the oxidation potential [35]. The tetrahexahedral Au nanoparticles have less positive oxidation potential than that of the elongated Au nanostructure and it was rationalized that the atoms at the step edges are chemically more active than that of plane surface [35]. It should be mention here that the particle size does not have major influence on oxidation potential when the size >35 nm [36]. The observed low oxidation potential of spherical Ag nanoparticles compared to polyhedral shape may be ascribed to the small size [37]. On the other hand, low oxidation potential of triangular nanoplate can be ascribed to its high reactivity.

3.3. Selectivity The selectivity of the assay was tested with coexisting species such as Co(II), Zn(II), Cd(II), Cu(II), Cr(III), Fe(II), Mn(II), Br− , PO4 3− , glucose, and glycerol at excess concentration (0.1 mM). No observable change in the spectral profile was noticed in the presence of these species (Fig. 5 and Fig. S8 in the Supporting Information), confirming the high selectivity of the sensing protocol. These interfering species are thermodynamically incapable of oxidizing Ag(0) to Ag(I) as the redox potentials of these species are lower than that of the Ag(I)/Ag(0) couple.

Fig. 5. Plot illustrating the selectivity of the optical sensing method toward H2 O2 . Change in the absorbance at 620 nm is plotted.

4. Conclusion A rapid and simple one pot synthesis of triangular Ag nanoplate using a NADH model compound and the colorimetric sensing of H2 O2 are demonstrated. The photoexcited BNAH, in its excited state efficiently reduces Ag(I) ions to Ag(0) and Ag(0) nucleate to give spherical nanoparticles. The spherical nanoparticles undergo lightinduced shape transformation to triangular plate shape through the formation of hexagonal plate. The citrate ions stabilize the nanoparticles. The triangular nanoplates undergo facile oxidative etching reaction with H2 O2 and yield Ag nanodisc. It is demonstrated that the oxidative etching reaction depends on the shape of the nanoparticles. The shape-dependent reactivity of Ag nanoparticles is exploited for the optical sensing of H2 O2 . The redox reaction between Ag(0) and H2 O2 strongly depends on the surface structure and shape of the nanostructures. The triangular Ag nanoplate shows the highest activity toward H2 O2 among others shapes tested here. Acknowledgments This work was financially supported by Council of Scientific and Industrial Research, New Delhi and Indian Institute of Technology Kharagpur. R.K.B. is a recipient of a UGC Fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jphotochem. 2013.07.005. References [1] E.C. Dreaden, M.A. El-Sayed, Detecting and destroying cancer cells in more than one way with noble metals and different confinement properties on the nanoscale, Accounts of Chemical Research 45 (2012) 1854. [2] U. Kreibig, M. Vollmer, Optical properties of metal clusters, in: Springer Series in Materials Science 25, Springer-Verlag, New York, 1995, pp. 50. [3] D.D. Evanoff, J.G. Chumanov, Size-controlled synthesis of nanoparticles. 2. Measurement of extinction, scattering, and absorption cross sections, Journal of Physical Chemistry B 108 (2004) 13957. [4] K.A. Homan, M. Souza, R. Truby, G.P. Luke, C. Green, E. Vreeland, S. Emelianov, Silver nanoplate contrast agents for in vivo molecular photoacoustic imaging, ACS Nano 6 (2012) 641. [5] J.E. Millstone, S.J. Hurst, G.S. Me’traux, J.I. Cutler, C.A. Mirkin, Colloidal gold and silver triangular nanoprisms, Small 5 (2009) 646. [6] L. Maretti, P.S. Billon, Y. Liu, J.C. Scaiano, Facile photochemical synthesis and characterization of highly fluorescent silver nanoparticles, Journal of the American Chemical Society 131 (2009) 13972. [7] S. Jradi, L. Balan, X.H. Zeng, J. Plain, D.J. Lougnot, P. Royer, R. Bachelot, S. Akil, O. Soppera, L. Vidal, Spatially controlled synthesis of silver nanoparticles and nanowires by photosensitized reduction, Nanotechnology 21 (2010) 095605.

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