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Size Control of Silver-Core/Silica-Shell Nanoparticles Fabricated by Laser Ablation Assisted Chemical Reduction Victor A. Ermakov, Ernesto Jiménez-Villar, José Maria Clemente da Silva Filho, Emre Yassitepe, Naga Vishnu Vardhan Mogili, Fernando Iikawa, Gilberto Fernandes De Sá, Carlos Lenz Cesar, and Francisco C. Marques Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04308 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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Size Control of Silver-Core/Silica-Shell Nanoparticles Fabricated by Laser Ablation Assisted Chemical Reduction Victor A. Ermakov1*, Ernesto Jimenez Villar2*, José Maria Clemente da Silva Filho1, Emre Yassitepe3, Naga Vishnu Vardhan Mogili4, Fernando Iikawa1, Gilberto Fernandes de Sá2, Carlos Lenz Cesar1,5, Francisco Chagas Marques1 1

Universidade Estadual de Campinas, IFGW, Campinas, SP, 13083-859, Brazil Universidade Federal de Pernambuco, DQF, Recife, CP, 50670-901, Brazil 3 Universidade Estadual de Campinas, IQ, Campinas, SP, 13083-859, Brazil 4 LNNano, CNPEM, Campinas, SP, 13083-970, Brazil 5 Universidade Federal do Ceará, DF, Fortaleza, CE, 60440-900, Brazil KEYWORDS. Size control, silver nanoparticles, assisted laser ablation. 2

ABSTRACT: Aqueous colloidal silver nanoparticles have a substantial potential in biological application as markers and antibacterial agents, and in surface-enhanced Raman spectroscopy (SERS) applications. A simple method of fabrication and encapsulation into an inert shell is of great im portance today in order to make their use ubiquitous. Here we show that colloids of Silver-Core/Silica-Shell nanoparticles can be easily fabricated by a laser ablation assisted chemical reduction method and their sizes can be tuned in the range of 2.5 to 6.3 nm by simply choosing a proper waterethanol proportion. The produced silver nanoparticles possess a porous amorphous silica shell that increases the inertness and stability of colloids, which decreases their toxicity compared to those without silica. The presence of a thin 2-3 nm silica shell was proved by EDX mapping. The small sizes of nanoparticles achieved by this method were ana lyzed using optical techniques and they show typical photoluminescence in the UV-Vis range, that shifts towards higher energies with decreasing size.

medical biophysics 15.

INTRODUCTION Metallic nanoparticles (NPs) have been used since the 5th century BC to control colors 1, and today, are used especially to enhance light-matter interaction, such as Surface Enhanced Raman Spectroscopy (SERS) 2 with Surface Plasmon Resonance (SPR) 3. Since ancient times many other applications have emerged including biological sensing and imaging 4, photothermal cancer therapy 5, enhanced photoswitchable bactericidal effects 6, enhancement of drug efficiency 7, light concentrators in photovoltaic 8 and in electronics 9. Metal NPs have been synthesized by different methods such as photochemical 10, laser ablation in a liquid environment 11– 13 electrochemical , and chemical reduction 14. The best results with this last technique have been achieved in nonaqueous and nonphysiological pH solution, although the collected NPs re quire further processing in order to be used in biology and

In 2008 Jimenez demonstrated a novel hybrid physicalchemical method based on a chemical reaction assisted by laser ablation 16 . This is a fast (2-5 min) and simple method to synthesize metal-core/dielectric-shell nanoparticles in one step and in an aqueous medium 17–19. This method, assisted laser ablation (ALA), is basically a chemical synthesis (red-ox), where one of the reactants (silicon) is supplied in nanometer dimensions by laser ablation. Laser ablation has been used as a powerful instrument to fabricate nanostruc tures in several environments: vacuum 20, an inert atmosphere 21, reactive atmosphere 22,23 and liquid 11–13. On the other hand, it is known that by controlling the kinetic reac tion, it could control the size and shape of metal nanoparti cles 24. This means that by controlling the kinetic of nanosil icon oxidation process supplied by laser ablation, it could control the metal-salt reduction process and consequently, the size and shape of metallic core.

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Figure 1. a – Layout of laser ablation assisted chemical reduction method for fabrication of silver nanoparticles covered by a silica shell; b – photo of the samples with different ethanol concentrations; c – absorption spectra; d – example of spectrum deconvolution (0% ethanol); e – linear depen dence of the maximum SPR peak position on ethanol concentration (purple dots, shadowed area shows confidence bounds of linear regression) and simulation of SPR peak of nanoparticles with 2 nm silica shell (2 layer) using MIE formulation 25.

In the current work we focus on the synthesis, by ALA in a water-ethanol solution, of colloidal silver NPs with a tun able size coated with a silica shell. The laser ablation process produces silicon nanoparticles that are subsequently oxidized in the water-ethanol solution, yielding silica nanoparticles. In a second step, the silver salt (AgNO 3) is added. The surface of these silica nanoparticles must be haves like a reducing agent resulting in the formation of sil ver nanoparticles coated with a silica shell. An advantage of this method is that on top of the metallic nanoparticle a sil ica shell is automatically formed with no need for any passi vation or preparative procedures. Additionally, the low cy totoxicity of the synthesis process of NPs covered by silica would facilitate its biological applications. Notice that most of the formation methods of the silica shell on a metallic particle are based on the Stober method 26 that involves toxic agents. The silica shell provides a high dispersibility of NPs 27 and inertness 28 , which has enabled their use in numerous applications 29,30 . Other works involving laser ablation by producing core-shell nanoparticles have been pub lished elsewhere 12,13.

In this paper we show how to control the size of silver nanoparticles produced by the method of laser ablation as sisted chemical reduction, by a simple addition of ethanol into milli-Q water. Using this technique, we succeeded in producing silver nanoparticles covered with a porous silica shell with silver core diameters varying from 2.5 to 6.3 nm and shell approximately 2-3 nm thick. EXPERIMENTAL We fabricated silver nanoparticles using a two-step physicalchemical route. The first step (physical) is laser ablation of the silicon target in a water-ethanol solution. For this we de signed a set-up at which laser passes through the transparent bottom of the cup and hits the target. The used geometry simplifies the adjustment of optimized focusing condition that is critical in this experiment 31 . Once done for the first sample it allows to produce at approximately equal condi tion a series of the samples by only changing solvent. We depict the preparation process schematically in Figure 1a. The monocrystalline silicon target (purchased from Virginia Semiconductor, 99.99999%) was immersed in 25 ml of

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milliQ water and spun at ~60 rpm. The target was then ab lated for 2 minutes with 1064 nm pulsed laser light that was focused on the target by a 50 mm cylindrical lens. Laser op tical path inside the solution was approximately 30 mm. A Nd:YAG laser (Quantel Brio) with a 9 ns pulse (20 Hz repe tition rate), at 100 mJ per pulse (resulting in a fluence of 2 J/cm 2), was employed. This results in a slight change to the solution’s transparency and pH from pH 7 down to pH 4.5. The change of solution´s transparency may be associated to the splashing effect during the laser ablation, which pro vokes the fragmentation of target surface, producing big sili con particles. These particles precipitate in a few minutes. At the second step (chemical), we separated 15 ml of the so lution immediately after laser ablation and added 150 µl of AgNO 3 (for analysis EMSURE, purchased at Merck) at 10 −2 M and 40 µl of Na 2CO 3 (purchased at Merck) at 10 −1 M. The addition of silver nitrate into the solution provoked a rapid color change from almost transparent to yel low, which indicates the formation of silver nanoparticles. Further addition of sodium carbonate normalizes the pH of the solution (resulting in pH 8). By our experience, col loidal suspension of silver nanoparticles at pH 8 remains un changed for many months. In order to control particle size, we partially substituted water with ethanol and prepared samples with different con centrations; i.e. 0, 10, 20, 25, 30, 40, 50, 75 and 100% of ethanol (gradient grade for liquid chromatography LiChrosolv purchased at Merck, purity >99.9%). Samples changed colors from clear yellow with 0% of ethanol to more brown ish or reddish yellow with 100% of ethanol ( Figure 1b). The samples were characterized by adsorption and photolumi nescence spectroscopy, transmission electron microscopy, X-ray diffraction techniques, and energy-dispersive X-ray spectroscopy. The ultraviolet-visible (UV-Vis) extinction spectra were measured with an Agilent Cary 60 UV-Vis spectrophotometer using quartz cuvettes with a 10 mm optical path. Trans mission electron microscopy (TEM) analysis was conducted in JEOL JEM 2100F TEM operating at 200 kV. Further En ergy Dispersive X-ray Spectroscopy (EDX) analysis was car ried out using state-of-the-art double C s -corrected FEI Titan cubed Themis TEM equipped with Super-X quad win dowless EDX detectors operating at 300 kV. The TEM sam ples were prepared by immersing a carbon-coated copper grid into solutions and dried. Size distribution histograms were obtained by a manual measurement of the particle size on Bright Field TEM images and by automatically counting them using free software (Gwyddion and ImageJ). Around 1000 particles were analysed for each sample. EDX data was gathered in Scanning TEM (STEM) mode in order to record Hypermaps (3d data cube) using QUANTAX software from Bruker. X-ray diffraction spectra were recorded on Rigaku. XRD samples were prepared by dropping 5 ml of solution onto a Si substrate placed on a hot plate at 100ºC. Micro-photoluminescence (PL) measurements were per formed using a He-Cd laser (325nm, 150 uW) as an excita tion light and detected by a 1/2m single monochromator

coupled with a Si-CCD (Andor). The 15X UV-objective was used to focus the laser beam and collect the luminescence. The PL was analyzed from colloidal solution in a quartz cu vette at room temperature. RESULTS AND DISCUSSION Optical absorption. Figure 1c shows the absorption spectra of the samples. A strong SPR peak located at around 400 nm shifts towards lower energies (longer wavelength) and its intensity diminishes as ethanol proportion is increased. We deconvoluted the spectra using three Voigt contours (pur ple contours on Figure 1d). The more intense band (~400 nm) that the red shift undergoes is associated with SPR of silver nanoparticles 3. Another bands located between 500600 nm are much weaker and broader, and must be associ ated with the agglomeration of the nanoparticles. Notice that, a decrease of zeta potential is expected as the ethanol proportion is increased 32 , which in turn might also favor an increase of the silica shell thickness. The third Voigt con tour is associated with inter band transitions 33 . Fitting revealed a linear dependence of the maximum of SPR peak po sition with ethanol proportion (Figure 1e, fitting results are listed in the Supporting Information). Size distribution . Figure 2a shows a typical image obtained using TEM and the inset shows a HR-TEM image of a silver nanoparticle with a diameter around 5 nm. The in terplanar distance is ~0.255 nm, showing that the core is crystalline with lattice constant of 0.440 nm which is very close to the reported for silver FCC elsewhere (0.408 nm). This slight deviation of the lattice constant could be associ ated to the microscope calibration and/or the typical in crease of the lattice constant when NPs diameter decreases considerably. We analyzed approximately one thousand particles for each sample and plotted their experimental proba bility density function on Figure 2b. LogNormal function fits the particle distribution better than Weibull and Gamma, so we applied it to the extract mode (most fre quent) and standard deviation values. X-ray diffraction . Figure 2c shows a typical X-ray diffraction pattern of the silver nanoparticles in a region from 36 to 46 degrees. It has two prominent diffraction lines attrib uted to metallic silver with face-cubic centered packingplanes (111) and (200) at 38.2 and 44.3 degrees respec tively. The lattice parameter calculated from XRD pattern is a=4.0857 Å in good agreement with the literature value a = 4.086 Å. The diffraction pattern is in good agreement with the literature report (JCPDS File No. 04-0783). The XRD spectra does not show any evidence of the presence of crys talline silica, suggesting that the silica shell is amorphous. The (111) and (200) peaks were deconvoluted by two Gaussian contours (Figure 2c, inset; fitting results are listed in the Supporting Information). Deconvolution using two peaks is necessary in order to distinguish the behavior of small sized particles (less than 10 nm in diameter) that are dominant in the distribution, according to TEM data, from bigger ones (greater than 10 nm). This is due to the fact

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Figure 2. a – Typical TEM image (inset – HR-TEM image of a particle prepared at 50% ethanol solution); b – experimental probability density plot of the particle diameters; c – XRD diffractogram of the sample prepared at 0% ethanol solution (inset – fittings using two Gauss contours); d – EDX maps of Ag, Si and O distribution and corresponding HAADF image of a 5 nm nanoparticle; e – linear dependences of average diameters extracted using the TEM technique (blue dots), crystallite size extracted from XRD using Scherrer's equation: in (111) plane – blue dots and (200) plane – red dots (dotted lines represent linear regressions, shadowed areas show confidence bounds of linear regressions); f – relative change of the particle diameters with ethanol concentration (experimental – blue dots; linear regression – blue dashed line) and relative change of concentration of oxy gen (OH) atoms in the solutions (red line); g – photoluminescence spectra of nanoparticles in water and ethanol.

that the intensity is proportional to the scattering volume (volume of a particle) which in its case is proportional to the third order of a particle diameter. Thus, an impact of the 25 nm diameter particle is 125 times greater than one of a 5 nm diameter particle (25 3/5 3=125). Even a small number of particles of a bigger size (a few per cent in each distribu tion) can completely hide the impact of small (dominant) sized particles. The deconvoluted Gaussian peak associated with the small particles is broadened and allows for estimat ing mean crystallite size using Scherrer's formula.

EDX mapping . The elemental composition of several particles was analyzed from among all the samples and it was found that they were covered by a SiO 2 layer. Figure 2d shows typical distribution maps of the next elements: Ag, Si and O. The elemental maps are shown separately and super imposed one on top of another (the HAADF image of the particles is also presented in Figure 2d). From Figure 2a (HRTEM), one can distinguish an amorphous coating with

about 2-3 nm thickness surrounding the silver nanoparticle. Encapsulation of silver nanoparticles into a silica shell im proves colloidal stability, diminishes toxicity of the silver 6 can protect the silver core from direct contact with, for example, luminescent molecules enhancing the lumines cence and avoiding luminescence quenching 34 .

Mechanism of size control . The assisted laser ablation must create Si particles of nanometric sizes 17 . These Si nanoparticles are very reactive. This results in the instanta neous oxidation and fragmentation of particles, leading to the formation of SiO 2 subnanoparticles. Several isomers of SiO 2H 2 may be produced on the surface of these SiO 2 subnanoparticles, due to the extremely high energies involved during the laser ablation process. The production and study of these isomers has been previously reported by using of electrical discharges 35. Silver ions might be reduced through the reaction with these SiO 2H 2 isomers (Ag +1 →Ag 0)

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and start growing in the form of silver nanoparticles 17. By decreasing the molar concentration of oxygen (decrease H 2O proportion) during SiO 2 formation the reduction rate of silver ions can be decreased due to a decrease of the SiO 2H 2 isomers production, which must lead to the forma tion of the smaller silver nanoparticles. The increase of the core-shell ratio in the silver nanoparticles , together with the slight rise of refractive index of solution (increase of ethanol proportion), could explain a slight shifting of the SPR band toward longer wavelengths (red dotted line on Figure 1c). This can be proven by the simulation of absorption efficien cies of core-shell nanoparticles (2 layer) using MIE formula tion 25 . However, the great redshift observed in the SPR band can not be explained by MIE theory, it could be ex plained by the agglomeration of silver-core/silica shell nanoparticles. Notice that zeta potential associated to the silica nanoparticles must decrease as ethanol proportion is increased 32 . In Figure 2f we plotted a relative concentration of oxygen atoms (together with hydrogen and carbon) in the solutions, and on the same plot we added a relative size reduction of the nanoparticles taken from TEM. One can see that the change in the nanoparticle diameter reflects a decrease of oxygen (OH) concentration. The latter could be an indicium of the direct influence of oxygen concentration on silver nanoparticle size. Notice that the formation of SiO 2H 2 isomers could be linked directly to oxygen (OH) concentration in the solvent 35.

make their application in biological labeling very promising. Demonstrated tunable SPR is interesting for SERS measure ment of the objects where the effect of luminescence quenching is undesirable, and can be avoided by the pres ence of a silica shell.

Luminescence . Figure 2g shows the photoluminescence spectra of nanoparticles fabricated in water and in ethanol with the peak maximum located at around 460 nm and 400 nm, respectively. These photoluminescence spectra are ra tionalized as the radiative decay of the surface plasmon in silver nanoparticles. The shift of the luminescence maxi mum to the higher wavelength with the increasing diameter of silver nanoparticles is consistent with previous results re ported in the literature 36 . CONCLUSIONS We produced silver nanoparticles coated with a porous amorphous silica shell by laser ablation assisted chemical synthesis. The method consists of ablation of the Si target in a water-ethanol solution, followed by addition of AgNO 3. We showed that the average diameter of silver nanoparticles can be tuned inside a range of 2.9 nm to 6.3 nm by control ling the water-ethanol proportion. SiO 2H 2 isomers on the silica nanoparticles surface were proposed as possible re ducing agents of the silver ions. Changes in nanoparticle size was associated with the concentration of oxygen (OH) atoms in the solution, which must directly decrease the oxy dation rate of silicon nanoparticles and the formation of SiO 2H 2 isomers. Consequently, a decrease in the reduction rate of silver ions is expected. A detailed study about the formation of SiO 2H 2 isomers and its chemical activity is called for, in order to discern accurately the formation mechanism of silver nanoparticles. The nanoparticles have a photoluminescence in the UV-vis region. The photolumi nescence of the nanoparticles, together with their other properties, like low toxicity and extremely small size, all

Victor Ermakov acknowledges FAPESP (grant 2013/26385-6). E.J.-V. acknowledges FACEPE (grant 0116-1.06/13). C. L. Cesar acknowledges the resources obtained from Instituto Nacional de Fotônica Aplicada à Biologia Celular - INFABIC (CNPq grant 573913/2008-0, FAPESP grant 08/57906-3); Biologia das Doenças Neoplásicas da Médula Óssea (FAPESP grant 11/51959-0); Bolsa de Produtividade (CNPq grant 312049/2014-5) and Centro de Óptica e Fotônica - CEPOF (FAPESP grant 05/51689-2). F.C. Marques acknowledges FAPESP (grant 2012/10127-5), INES/INCT/CNPq (grant 554336/2010-3), CNPq (grant 407887/2013-0), CAPES, and Lamult/IFGW/Unicamp.

ASSOCIATED CONTENT Supporting Information.

AUTHOR INFORMATION Corresponding Author * [email protected], [email protected]

Present Addresses Author Contributions V.A.E and E.J.-V. made substantial contribution to the work. All authors participated in drafting the article and revising it critically. All authors have given approval to the final version of the manuscript.

Funding Sources FAPESP (grants 05/51689-2, 08/57906-3, 11/51959-0, 2012/101275, 2013/26385-6); CNPq (grants 573913/2008-0, 554336/2010-3, 407887/2013-0, 312049/2014-5); FACEPE (grants 0116-1.06/13, APQ-0071-1.06/14).

Notes

ACKNOWLEDGMENT

ABBREVIATIONS SERS surface enhanced Raman spectroscopy; SPR surface plasmon resonance; CVD chemical vapor deposition; ALA assisted by laser ablation; NPs nanoparticles; TEM transmission electron microscopy, HR-TEM high resolution transmission electron microscopy, XRD Xray diffraction.

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Aqueous colloidal silver nanoparticles have a substantial potential in biological application as markers and antibacterial agents, and in surface-enhanced Raman spectroscopy (SERS) applications. A simple method of fabrication and encapsulation into an inert shell is of great importance today in order to make their use ubiquitous. Here we show that colloids of Silver-Core/Silica-Shell nanoparticles can be easily fabricated by a laser ablation assisted chemical reduction method and their sizes can be tuned in the range of 2.5 to 6.3 nm by simply choosing a proper water-ethanol proportion. The produced silver nanoparticles possess a porous amorphous silica shell that increases the inertness and stability of colloids, which decreases their toxicity compared to those without silica. The presence of a thin 2-3 nm silica shell was proved by EDX mapping. The small sizes of nanoparticles achieved by this method were analyzed using optical techniques and they show typical photoluminescence in the UV-Vis range, that shifts towards higher energies with decreasing size. Abstract 239x99mm (72 x 72 DPI)

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a – Layout of laser ablation assisted chemical reduction method for fabrication of silver nanoparticles covered by a silica shell; b – photo of the samples with different ethanol concentrations; c – absorption spectra; d – example of spectrum deconvolution (0% ethanol); e – linear dependence of the maximum SPR peak position on ethanol concentration (purple dots, shadowed area shows confidence bounds of linear regression) and simulation of SPR peak of nanoparticles with 2 nm silica shell (2 layer) using MIE formulation 25⁠ . Fiigure 1 422x305mm (72 x 72 DPI)

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a – Typical TEM image (inset – HR-TEM image of a particle prepared at 50% ethanol solution); b – experimental probability density plot of the particle diameters; c – XRD diffractogram of the sample prepared at 0% ethanol solution (inset – fittings using two Gauss contours); d – EDX maps of Ag, Si and O distribution and corresponding HAADF image of a 5 nm nanoparticle; e – linear dependences of average diameters extracted using the TEM technique (blue dots), crystallite size extracted from XRD using Scherrer's equation: in (111) plane – blue dots and (200) plane – red dots (dotted lines represent linear regressions, shadowed areas show confidence bounds of linear regressions); f – relative change of the particle diameters with ethanol concentration (experimental – blue dots; linear regression – blue dashed line) and relative change of concentration of oxygen (OH) atoms in the solutions (red line); g – photoluminescence spectra of nanoparticles in water and ethanol. Figure 2 423x317mm (96 x 96 DPI)

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