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www.nature.com/scientificreports SCIENTIFIC RE P ：RTS Received：25 November 2015 Accepted：04 March 2016 Solvent-surface interactionscontrol the phase structure in laser-generated iron-gold core-shellnanoparticles Published： 23 March 2016 Philipp Wagener，Jurij Jakobil， Christoph Rehbock，Venkata Sai Kiran Chakravadhanula2，tClaas Thede3， Ulf Wiedwaldf， Mathias Bartsch4， Lorenz Kienle² & Stephan Barcikowskil This work highlights a strategy for the one-step synthesis of FeAu nanoparticles by the pulsed laserablation of alloy targets in the presence of different solvents. This method allows particle generationwithout the use of additional chemicals；hence， solvent-metal interactions could be studied withoutcross effects from organic surface ligands. A detailed analysis of generated particles via transmissionelectron microscopy in combination with EDX elemental mapping could conclusively verify that thenature of the used solvent governs the internal phase structure of the formed nanoparticles. In thepresence of acetone or methyl methacrylate， a gold shell covering a non-oxidized iron core was formed，whereas in aqueous media， an Au corewith an Feg04shell was generated. This core-shell morphologywas the predominant species found in >90% of the examined nanoparticles. These findings indicatethat fundamental chemical interactions between the nanoparticle surface and the solvent significantlycontribute to phase segregation and elemental distribution in FeAu nanoparticles. A consecutiveanalysis of resulting Fe@Au core-shell nanoparticles revealed outstanding oxidation resistanceand fair magnetic and optical properties. In particular， the combination of these features with highstability magnetism and plasmonics may create new opportunities for this hybrid material in imagingapplications. One focus in nanotechnology is to combine nanoparticle properties， such as nanomagnetism， plasmonics， andthiol-based conjugation chemistry， to generate multifunctional nanomaterials. This can be achieved by combin-ing two or more metal components into a single alloy or core-shell nanoparticle23. For biomedical applications，the gold-iron system is particularly interesting4-7. In this context， iron-based particles are relevant because of theirmagnetic properties that are applicable for magnetic resonance imaging (MRI)& and thermotherapy， whereasgold is a promising material for its oxidation resistance， optical properties ， and ability to be easily functionalizedby thiolated biomolecules. Furthermore， the concept of dual MRI/optical imaging has been discussed in theliteraturel2.13. However， the combination of gold and iron within a single nanoparticle is non-trivial， as the phasediagrams of gold and iron are complicated and include a broad miscibility gap14. In addition， non-noble iron alsosuffers from oxidation， making the controlled formation of FeAu alloy nanoparticles even more challenging.Next to particle composition， the internal phase structure of alloy nanoparticles is also a critical determinantcontrolling particle properties. In the case of medical applications， a core-shell structure15-19， comprising an inertnoble metal shell (such as gold) protecting a reactive magnetic core (such as iron) from oxidation， is the mostpromising strategy20. Conventional syntheses of binary nanoparticles are based on the co-precipitation of metalsalts2l or reverse micelle methods13，22. In these cases， the chemical synthesis of Fe@Au core-shell nanoparticles Technical Chemistry I and Center for Nanointegration Duisburg-Essen (CENIDE)， University of Duisburg-Essen，Universitaetsstrasse 7， 45141 Essen，Germany.2AG-Synthesis and Real Structure， Institute for Materials Science，Faculty of Engineering， Kiel University， Kaiserstrasse 2， 24143 Kiel，Germany. AG-Inorganic Functional Materials，Institute for Materials Science， Faculty of Engineering， Kiel University， Kaiserstrasse 2， 24143 Kiel， Germany.“Facultyof Physics and Center for Nanointegration Duisburg-Essen (CENIDE)， University of Duisburg-Essen，Lotharstrasse1，47057 Duisburg，Germany. Present address： Helmholtz Institute Ulm， Karlsruhe Institute ofTechnology， Hermann--von-Helmholz-Platz 1， 76344 Eggenstein-Leopoldshafen， Germany. Correspondence and requests for materialsshould be addressed to S.B. (email： stephan.barcikowski@uni-due.de) can be achieved； however， inchoate oxygen-permeable Au shells formed， which led to slow oxidation ofthe Fecores within several days2. However， chemical synthesis routes are frequently complicated by numerous factors：I) a lack of suitable educts for specific synthesis approaches， II) impurities from precursors， and III) incom-plete conversion into uniform particles because of different reactivities and chemical equilibria22，23. More detailedinformation concerning the chemical synthesis of multi-material nanoparticles can be found in a recent reviewby Cortie and McDonagh24. Aside from chemical methods， several studies have also reported on the utilization of physical methodsfor FeAu nanoparticle synthesis. Amram et al. showed the synthesis of Fe@Au core-shell nanoparticles by thesolid-state dewetting of thin Fe-Au bilayer films deposited on a sapphire substrate， and Velasco et al. preparedFeAu alloy nanoparticles by inert-gas condensation for biomedical applications2. Among the physical synthesisroutes， pulsed laser ablation in liquids (PLAL) is one of the most promising methods for nanoparticle synthesis，as it produces pure and uniform alloy nanoparticles and may efficiently overcome some ofthe problems arisingduring chemical synthesis2. In this context， PLAL is a well-established method， and colloidal nanoparticles fromnumerous binary alloy targets， including AgAu， FeNi， NiTi， PtIr， and FeAu， in different liquids have been fabri-cated by this technique27-34. This method allows the generation of surfactant-free nanoparticle colloids withoutthe use of chemical precursors or toxic preservatives， which could be beneficial for potential biomedical appli-cations. The formation of solid-solution alloy nanoparticles by laser synthesis has been previously reported2731，including detailed studies of particle morphology in the case of monophasic solid-solution AgAu particles3. Agold-iron nanoparticle system made by laser ablation in solution was intensively studied by Amendola et al.28-30，35.They showed how to combine the optical properties of gold， such as surface-enhanced Raman scattering (SERS)，with magneto-responsive iron and iron oxide in ligand-free nanoparticles. An example of these materialspotential for biomedical applications was given by the magnetic sorting of murine macrophages with enclosedAuFeO-nanoparticles and the subsequent SERS-imaging of the sorted cells36. In another approach， the impactof the surrounding medium on the morphology of laser-generated FeAu nanoparticles was studied. Here， in eth-anol， non-segregated plasmonic alloy nanoparticles were obtained； however， oxide formation was reported afterthe addition of 0.2% of H，O or H，02 to ethanol. Although the formation of FeAu alloy particles via PLAL has been previously studied， a detailed understand-ing of the formation mechanism and a clear correlation between the composition of the surrounding medium andparticle internal phase structure remain lacking. To fill these gaps， this study systematically examines the PLALof FeAu targets in the presence of aqueous and organic media. Detailed high-resolution transmission electronmicroscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDX) analyses are utilized to probe the inter-nal structure and composition of the nanoparticles. Methods The laser-generated nanoparticles were obtained by using a femtosecond laser (Spitfire Pro， Spectra-Physics) witha central wavelength of 800 nm， a pulse duration of 120 fs， a pulse energy of 300 uJ， a repetition rate of 5 kHz， anda focal distance of 150 mm. The target (Fe44Aus6 alloy， custom made by the“Research Institute for Noble Metalsand Metal Chemistry， Schwaebisch Gmuend， Germany) was placed at the bottom of a vessel filled with 4mLof solvent (i.e.， deionized water， technical-grade acetone with 99.5% purity，or methyl methacrylate (MMA)，purchased from Sigma-Aldrich with 99% purity). The liquid layer above the target was 5 mm thick. A 5-axismanipulator moved the vessel with a speed of 1 mm/s in a spiral pattern for a total laser ablation time of 10 min.The influence of the pulse duration was investigated using a picosecond pulsed laser (Atlantic 532， Ekspla) witha wavelength of 1064 nm， a pulse duration of 10 ps，a pulse energy of 160 uJ， a repetition rate of 100kHz， and afocal distance of 100 mm， and a nanosecond pulsed laser (PowerLine 20 E， Rofin) with a wavelength of 1064nm，a pulse duration of 8 ns， a pulse energy of 0.8 mJ， a repetition rate of 15 kHz， and a focal distance of 100 mm. Thehydrodynamic size and zeta potential of the resulting colloidal solutions were characterized by dynamic lightscattering (DLS) using a Zetasizer (ZS， Malvern). High-resolution transmission electron microscopy (HR-TEM)was performed using a Tecnai F30 G2 ST instrument equipped for EDX analysis. Further elemental mappingusing energy-filtered TEM with a Gatan image filter Tridiem was also performed to confirm the results. Thefunctionality of the nanoparticles in terms of their optical and magnetic properties was analyzed. The opticalproperties were studied within the wavelength range of 350-800 nm using ultraviolet/visible (UV/Vis) absorp-tion spectroscopy (Shimadzu 1650) using a quartz cuvette (Helma Analytics) with a path length of 10 mm. Themagnetic properties of the nanoparticles produced by ablation in acetone were studied using a vibrating samplemagnetometer (VSM， Lakeshore Model 7300). A sample for VSM measurements was prepared by drying 100 uLof a nanoparticle dispersion (mass concentration= 374 ug/mL) on pieces of filter paper that were 5 mm ×5 mmin size. Focused ion beam (FIB) techniques (FEI Helios Nanolab combined Focused Ion Beam/SEM) were usedfor the preparation of a thin lamella containing solid nanoparticles. The FIB-milled nanoparticles were analyzedusing TEM-EDX techniques and an FEI Tecnai F20 instrument. Details concerning the FIB techniques and themilling procedure of the solid nanoparticles are presented in the Supplementary Information. The nanoparticles’chemical stability was probed using chemical etching protocols involving hydrochloric acid. First， the colloidalnanoparticles were exposed to 10% HCl for 1 h and subsequently magnetically separated and washed with deion-ized water. In the second step， etching was conducted with concentrated HCl (37%， for 1h)， and the samples werewashed three times and magnetically separated. Results and Discussion The laser ablation of FeAu targets was conducted in three different solvents-water， acetone， and MMA-and inall cases， stable nanoparticle colloids were obtained. The stability was probed using time-resolved UV-Vis spec-troscopy measurements. The spectra clearly reveal that the extinction at入=520 nm of the colloids formed in ace-tone decreased by <8%over a period of 21 days， indicating good colloidal stability. Furthermore， the extinction Figure 1. Size distributions of FeAu alloy nanoparticles generated in (a) acetone， (b)MMA， and (c) water.The number-weighted particle size distributions were fitted with a log-normal function. (d) Hydrodynamicdiameters (dh)， zeta potentials (E)，and Feret diameters (df) of FeAu nanoparticle colloids in different solvents(E could not be measured in MMA because of high viscosity). The df values were obtained from the numbermean values of the log-normal fitting functions. recorded in the near-infrared (NIR) range (X=800 nm) was completely unchanged， verifying that no additionalagglomerates， which are prone to show intensive scattering in this spectral regime， formed in the respective timeperiod (Fig. S13 in the Supplementary Information). Subsequently， the generated colloids were characterizedusing DLS and TEM. The resulting Feret diameters， hydrodynamic diameters， and zeta potentials are shown inFig. 1d. Furthermore， the particle size distributions obtained from the TEM images are depicted in Fig. la-c.Although the hydrodynamic diameters (d，) of the colloids generated in all three solvents were very similar， theFeret diameters (dr) significantly deviated. In all cases， dh was larger than dp possibly because particle analysistechniques， such as DLS， overestimate larger particles and small agglomerates in solution and cannot properlycharacterize small particle fractions in the presence of large ones. In addition， the TEM images (Fig. 1) seemto indicate the presence of agglomerated primary particles. Based on these data， whether these agglomerationprocesses are artefacts of drying on the TEM grids or whether agglomerates are present in solution remainsunclear. However， a contribution of agglomerates formed during the ablation process to the hydrodynamic diam-eter measured via DLS cannot be excluded. These agglomerates would be detected as individual particles by DLSand may explain why d was larger than d. Furthermore， the particle sizes (dr) of FeAu in acetone were significantly larger than those found in MMA andwater. Particle size distributions during PLAL are dominated by particle growth-quenching mechanisms， induc-ing the stabilization of reactive particle surfaces by solvent molecules. In aqueous solutions， this phenomenon isgoverned by anion adsorption38，39 in the case of noble metals， such as Au. However， in non-noble metals， such asiron， surface oxidation and the formation of iron hydroxide shells have been reported to be key factors drivingparticle stabilization’. In the presence of organic solvents， the formation of enolates and alcoholates， which maypolymerize on the particle surface， can induce stabilizing effects40，41. A detailed discussion of this phenomenon inthe context of particle phase structure can be found below. The zeta potentials of the FeAu nanoparticles synthesized in acetone and an aqueous solution deviated signif-icantly， with the acetone-based solution exhibiting more negative values. Although highly negative zeta poten-tial values are typical for metal colloids obtained by PLAL27，4243， the pronounced differences in zeta potentialsbetween the colloids obtained in water and acetone indicate deviating surface charge densities. This may beevidence of pronounced differences in the particle surface chemistry caused by different surface compositions.Because further interpretation of these data would require a detailed knowledge of the particle's elemental com-position， the internal phase structure of the nanoparticles was further characterized using high-angle annulardark field scanning transmission electron microscopy (HAADF-STEM) in combination with EDX. Figure 2. (a)SETM-HAADF EDX line scan of an intermetallic (solid-solution) nanoparticle.(b) STEM-HAADF images of intermetallic nanoparticles that were laser-generated in acetone. b 50nm 50 nm 20nm Figure 3. (a) STEM-HAADF images providing z-contrast (left and right images) and STEM-EDX elementalmaps of nanoparticles from the laser ablation of FeAu in acetone (for MMA， see Supplementary Fig.S2)，confirming the presence ofthe non-oxidized iron core and gold shell. Line scans (b) of the nanoparticle in (a)showing the core-shell nature. The analysis of the morphology revealed interesting properties. In all solvents， bimetallic FeAu nanoparticleswere generated， which showed a pronounced core-shell structure. Intermetallic nanoparticles with homogeneouselemental distributions formed less frequently (Fig. 2) and were only found in <10% of all examined particles.However， an elemental analysis of FeAu nanoparticles by EDX on the single-particle level revealed an atomiccomposition of Fe47Aus3 (average of five point measurements)， which was similar to that of the target material(Fe4Aus6， Supplementary Table S1 and Fig. S9). Therefore， although the overall composition of the particlesremained similar in all experiments and was close to the composition of the corresponding target (Fe44Au56)， thestructure and morphology of the core-shell material clearly differed depending on the solvents applied duringlaser ablation. In the case of PLAL in acetone and MMA， a gold shell formed around a non-oxidized iron core(Fig. 3a，b， and Supplementary Fig. S2). In contrast to chemically derived Fe@Au nanoparticles， which weresubject to rapid oxidation of the iron core， in the present study， EDX revealed no oxygen in the metal core， evenafter several weeks of storage. Thus， the Au shell of the laser-generated nanoparticles protects the reactive Fe coreagainst oxidation. These findings were clearly confirmed by the EDX analysis (Fig.3b). The majority ofthe binary 100 Core-shell nanoparticle Au-M -1Fe-K 2.0 4.0 6.0 8.0 2.0 4.0 6.0 8.0 Energy(keV) Energy(keV) Figure 4. Analysis of core-shell nanoparticles that were laser-generated in acetone. (a) Line scan and EDXsignal of a core-shell nanoparticle. (b)STEM-HAADF image of an Fe@Au core-shell nanoparticle with regionsof EDX analysis indicated (shell： regions 1-3， predominantly Au， core： region 4， composed of Fe and Au. Forthe detailed EDX analysis， see Supplementary Fig.S6).(c) STEM-HAADF image of an FIB-milled Fe@Au core-shell nanoparticle.(d) Histograms of the shell thickness and core diameter of laser-generated Fe@Au core-shellnanoparticles. Average values and peak maxima indicate a mean core diameter of 15 nm and a shell thicknessof 3 nm. (e) Representative EDX point measurements indicating the presence of pure Au in the shell (region b1)and a mixture of Au and iron in the core (region b4). nanoparticles exhibited a mean shell thickness of3 nm and a core diameter of 15 nm (Fig. 4d). These findingswere further verified by line scans (Fig. 4a and Supplementary Fig. S8). The differences between the core-shell andhomogeneous alloy nanoparticles could be further clarified by normalizing the element-specific signal to the totalsignal (Supplementary Figs. S4and S5). These results conclusively demonstrate that the majority of the examinedparticles formed a defined core-shell nanostructure with iron enrichment in the core and gold enrichment inthe shell without any traces of oxygen. However， it should be noted that the elemental segregation might not beabsolute. Additional EDX analysis of different nanoparticle regions (Fig. 4b) clearly revealed that no significantquantities of iron were present in the shell (Fig. 4e， Supplementary Fig. S6， regions 1-3)； however， naturally， bothiron and gold were found when the whole particle， shell and core， (region 4) were analyzed.Based on these meas-urements， we cannot exclude the presence of low amounts of gold in the core， but the formation of an iron-richcore material was clearly indicated. These findings are in good agreement with recent literature， where an Fe-Aucore-shell structure synthesized by chemical methods was reported based on similar EDX measurements44，45.Tovalidate this unique structure of a crystalline gold shell protecting an elemental iron core， we measured the EDXline scans of a focused ion beam-prepared cross-section through some nanoparticles (Supplementary Figs. S7and S8). This technique confirmed the presence of a characteristic elemental distribution of a core-shell phasestructure after FIB cutting， with a sharp intra-particle interface between the shell and the core imaged， as shownin Fig. 4c. However， it should be noted that， based on the line scans， the presence of low amounts of iron in theshell and low amounts of gold in the core cannot be excluded， although a significant elemental enrichment isobvious. In a recent publication， Scaramuzza et al. applied X-ray photoelectron spectroscopy and showed that anFeAu shell containing oxidized iron atoms also contributed to the stabilization of an iron-rich core. As the EDXmeasurements conducted in this study were not surface sensitive， the formation of an FeAu instead of a pure Aushell cannot be excluded. To demonstrate the possible influence of the laser pulse duration on the formation of core-shell nanoparticles，FeAu nanoparticles were generated in acetone with three different laser sources and pulse durations of 120 fs， 100 80- 60- 40 20 50 Figure 5. HAADF-STEM images and EDX line scans of Fe@Au core-shell nanoparticles generated byablation with different laser pulse durations in acetone.(a) 120fs， (b) 10 ps， and (c) 8ns. 10 ps，and 8ns. HAADF-STEM measurements combined with EDX line scans were conducted for representativeparticles within the samples. Interestingly， the morphology determined by all three experimental setups revealedthe formation of Fe@Au core-shell nanoparticles (Fig.5). In this context， it should be noted that the pulse dura-tion was not the only parameter altered during the experiments. Indeed， the laser fluence and repetition rate and，hence， nanoparticle productivity， significantly deviated between the various setups. Nonetheless， these findingsclearly indicate that the formation of core-shell structures during PLAL of FeAu alloys is a universal phenomenonduring PLAL and is not limited to ultrashort (femtosecond) pulses. A very significant characteristic of the obtained Fe@Au core-shell nanoparticles fabricated in acetone andMMA is the absence of oxygen within the particle core. To probe the extent to which the Au shell may protectthe iron-rich core from oxidation， representative samples (laser-synthesized in acetone) were exposed to highlycorrosive， concentrated hydrochloric acid to oxidize the iron. Etching the core-shell nanoparticles using hydro-chloric acid did not result in any significant changes in the particle's internal phase structure (Supplementary Fig.S12)， as verified by EDX. Therefore，we can conclude that the gold shell is dense enough to protect the reactiveiron core against oxidation. This feature is beneficial for applications where high particle stability is of paramountimportance，e.g.， during biomedical imaging. In this context， the laser-generated Fe@Au core-shell particles aredefinitely superior to their chemically derived analogs， which exhibit iron core oxidation over time. Thus， PLAL of FeAu targets in acetone and MMA yields Fe@Au core-shell nanoparticles； however， whencomparable experiments were performed in water， the resulting phase structure was fundamentally different.As shown in Fig. 6， a reverse nanostructure composed of a gold core surrounded by an oxidized iron shell wasformed. STEM-HAADF elemental maps (Fig.6a) show the presence of a gold core and a shell enriched with ironand oxygen. Selected-area diffraction studies and the fast Fourier transformation results supported the exist-ence of the oxidized iron shell (Supplementary Fig. S3)， which was primarily composed of Fe；04(Fig.6b). Inaddition， no pronounced segregation of gold and iron in the core was noted， indicating that solid-solution nan-oparticles may form in the core under these conditions. Electrochemically， the redox potential for elementaliron (-0.447V)46 and the hydrogen overpotential for bulk iron (0.40V)47 indicate that water， in the absence ofdissolved O， is capable of (slowly) oxidizing iron； however， it is unlikely that pure water at a neutral pH couldbe responsible for the rapid and extensive particle oxidation observed in these experiments. Consequently， it ishighly likely that dissolved molecular oxygen further increases the oxidative potential of the liquid. To that end，outgassing can be considered as a suitable method for reducing particle oxidation. However， as the Amendolagroup recently observed during PLAL of an Fe7Au73 alloy in water， the majority of Fe oxidized during lasersynthesis， independent of previous degassing procedures with N2， Ar， or CO2. Because oxygen removal byinert-gas treatment of water is never complete (typically 0.5-2 mg/L residual oxygen) andd hbeeccaaiuissee the nano-particle mass concentration is practically limited by the colloidal stability threshold (typically less than 0.5 mg/mL for ligand-free PLAL-derived gold in water)， an iron mass fraction that can be oxidized by residual oxygenafter degassing will remain. In addition， it should be noted that the water， acetone， and MMA were not degassedand that the solubility of oxygen in acetone is seven times higher than that in water (solubility： oxygen in water：5.99 cm’/L； oxygen in acetone： 45 cm’/L)48. Thus， molecular oxygen was present in all liquids during the PLALexperiments. Hence， if dissolved oxygen was the main effector driving the particle internal phase structure， itshould be similar in all samples. Consequently， it is likely that the completely different outcomes of intra-particlephase segregation using water or organic solvents are not determined by dissolved oxygen but are more likely tobe the result of the intrinsic properties of the respective liquids. a Figure 6. (a) STEM-HAADF images (left and right images) and STEM-EDX elemental maps of thenanoparticles formed by the laser ablation of FeAu in water showing the oxidized shell containing iron aroundthe gold core； (b) selected-area diffraction pattern confirming the presence of Fe，O4. To understand the possible reasons for the solvent-dependent deviations of particle properties， a mechanisticperspective regarding the particle formation process during PLAL is beneficial. During PLAL， the alloy targetcontaining Fe and Au is ablated and the elements are trapped inside a laser-induced plasma plume， which isfollowed by the formation of a cavitation bubble49，50. During the PLAL process， plasma formation occurs on ahundreds of nanoseconds timescale after the pulse interacts with the target surface， whereas the lifetime of thecavitation bubble is in the order of 100-200 us depending on pulse energy51，52. Recent experiments have shownthat the geometry of the bubble is dominated by the plasma plume3. Because the plasma plume is hot (up to7000K) and subject to strong spatial confinement by the liquid， the Au and Fe atoms released from the target arehomogeneously mixed， and segregation is minimized. Consequently， the initial atomic composition inside theplasma and the initial bubble is solely dominated by the target composition. This is supported by the fact that theheats of evaporation for Au (356kJ mol-) and Fe (355kJ mol-) are very similar； thus， preservation of the stoi-chiometry can be assumed2. In this context， it should be noted that the PLAL process significantly differs frompulsed laser deposition， where the composition depends on the angle between the laser beam and the target. Ina consecutive process， the cavitation bubble expands， whereas the pressure and temperature drastically decreaseThis process occurs on a microsecond timescale and induces the formation of initial nanoparticles by crystalli-zation and nucleation54. It should be noted that this fast nucleation process is purely controlled by kinetics andthat the resulting particles possess a stoichiometry that， next to temperature and pressure， is controlled by theatom concentrations inside the cavitation bubble and， hence， the target composition. Previous experiments haveshown that medium components from the solvent， e.g.， ions， can be found in the plasma plume55. Consequently，solvent effects could occur during the early stages of the particle-formation process and may contribute to apotential ion-induced size-quenching effect38，39. However， an effect of solvents on particle stoichiometry has notbeen reported in the literature and was not observed in our study as the EDX analysis of the FeAu nanoparticlesand the targets (Supplementary Table Sl and Fig. S9) revealed that similar nanoparticle compositions (i.e.， Fe：Auratios) existed in all solvents. This preservation of the target composition has been previously observed in severalnanoparticle systems e.g.， PtIror AgAu32.34. Although the stoichiometry of the target material is generally preserved during PLAL of alloys and is pre-defined during the initial stages of the particle formation process (kinetically controlled)， the internal phasestructure of the nanoparticles has been shown to differ depending on the solvent. The De Giacomo group con-cluded in their review that the dynamics of cavitation bubble shrinking and the transfer of the nanoparticles intosolution after bubble collapse allow nanoparticles to be formed in quasi-constant thermodynamic conditions2.Consequently， phase segregation and the formation of core-shell structures most likely occur in the solutionunder thermodynamic control on a timescale far longer than the cavitation bubble lifetime. An illustration of theproposed particle-formation mechanism can be found in Fig. 7. To further understand the thermodynamically controlled formation of different internal phase structures indifferent solvents， we compared our findings with data recently reported by Malviya and Chattopadhyay， whoexamined the synthesis of Cu-Ag alloy nanoparticles by PLAL in an aqueous， polymer-doped medium. Theyreported Cu-concentration-dependent morphological transitions in the nanoparticles from a defined two-phasenanostructure to a structure with random segregation and， finally， to a core-shell structure. Their hypothesizedformation mechanism was rationalized through the thermodynamic modelling of the free energy of phase mixingand the wettability of the alloy phases. Similar to the Cu-Ag system， the iron-gold system shows a quite complexphase diagram， including several phases and miscibility gaps. In the FeAu bulk phase diagram， at room temper-ature and a composition of approximately 50：50， which corresponds to the target and particle compositions usedin this study， the most thermodynamically stable state is complete phase separation， which can be realized via acore-shell structure. Consequently， STEM-HAADF elemental maps of the nanoparticles that were laser-generated a Plasma state b Kinetically controlled nucleation c Thermodynamically controlledsegregation plasma Au●Fe plume target Figure 7. Steps of the nanoparticle-formation mechanism after laser pulse absorption.(a) Plasmaformation and initial cavitation bubble formation， (b) expansion of the cavitation bubble into the liquid andrapid cooling of the ablated matter， and (c) cavitation bubble collapse and nanoparticle release into the liquid. in acetone (Fig. 3a)， together with the EDX line scans (Fig. 3b) and the electron microscopy images acquired afterFIB cutting (Fig. 4c) the particles， confirm the formation of this structure. This finding is in good agreement withthe density functional theory (DFT) calculations by Wang and Johnson， who predicted exactly this core-shellpreference for FeAu binary nanoparticle systems8. Although this hypothesis conclusively explains why the phase separation and formation of core-shell struc-tures occur， it cannot explain why gold is predominantly formed on the outside ofthe particle in organic solvents，whereas the reverse composition is generated in water. Here， the interaction of the nanoparticle surface and thesolvent， namely， the nanoparticle surface chemistry， may be an important driving force. In organic solvents， goldsurfaces are probably formed on the outside of the particle because their interactions with the solvent are favored.Previous experiments reported in the literature indicated that noble metal colloids obtained by PLAL in acetoneor ethanol were stabilized by adsorbed enolates or alcoholates， respectively， whereas non-polar solvents couldnot provide any stabilization40. The presence of enolates on PLAL-generated gold nanoparticles in acetone waspreviously demonstrated by SERS41. Both organic solvents (i.e.， acetone and MMA) used in the present studycontain a keto group. Based on these previous findings from the literature， they may undergo self-polymerizationon the gold surfaces of the Fe@Au particles， contributing to the stabilization of these particles in the respec-tive solvents. This self-polymerization process of solvents is more strongly pronounced for a3-unsaturated car-bonyl compounds， such as MMA， compared with acetone. This could explain why particle stabilization and sizequenching are more efficient in MMA than in acetone and result in reduced particle diameters in the presence ofMMA (Fig. ld). However， a reverse morphology， with iron oxide on the outside， may be thermodynamically lessfavored in MMA and acetone， possibly because the potential stabilization mechanisms for hydrophobic gold， e.g.，solvent polymerization， may be thermodynamically less favorable for iron oxide. However， the exact nature of thesolvent-metal interactions cannot be elucidated based on the current experimental data. In contrast， in an aqueous solution， a completely different internal phase structure with an iron oxide shelland a gold core forms. As iron oxide is more hydrophilic than elemental gold， its formation on the particle surfacein the presence of water is favored. It is well known that metal nanoparticles in aqueous environments form apH-dependent equilibrium between M-O-/M-OH/M-OH，+(M=metal)， which is the main effector dominatingtheir surface chemistry59，60. Based on this theory， we can assume that the chemical interactions between nanoparticles and solvents arean important driving force governing the particle morphology of FeAu in particular-Fe50Au50 bimetallic nan-oparticles.This leads to significantly different surface chemistries with either an iron oxide shell or a gold shell，which is probably responsible for the deviating surface charge densities and， hence， the different zeta potentialsobserved in the different solvents (Fig. 1d). A similar mechanism was previously reported by Mayrhofer et al.，who observed a surface segregation of intermetallic PtCo nanoparticles that formed core-shell structures in thepresence of carbon monoxide. Because the adsorption enthalpy of carbon monoxide with Pt is more negativethan that with Co， Pt segregates on the surface of the nanoparticles， resulting in core-shell structures with a Cocore and a Pt shell. Further examples of the surface segregation of bimetallic nanoparticles resulting from chem-ical reactions at the surface can be found in the literature62-64 Subsequently， the optical and magnetic properties of these novel hybrid FeAu nanoparticles were evaluated.Representative UV-Vis spectra are shown in Fig. 8 and depict a broad extinction in the visible spectral rangesuperimposed by a gold plasmon resonance peak. The peak position can be visualized more clearly by taking thefirst derivative， as shown in the inset image in Fig. 8. For all used solvents， the plasmon resonance band is locatedat approximately 530 nm. Although these surface plasmon resonance (SPR) peaks are relatively weak comparedto those of pure gold nanoparticles， the signals in water are significantly larger than those in acetone and MMA.These findings are in accordance with those of previous experiments by Zhang et al.44. This study also reporteda dampening of the SPR maximum in the case where Au was alloyed with Fe in a core-shell structure， and theseexperimental data were in good agreement with simulations. It should be noted that the observed spectra for theFe@Au system significantly deviate from those of Fe；04@Au65， Co@Au66，and FePt@Au7， where more intense，red-shifted SPR peak signals were observed. However， compared with different materials， both the particle sizesand shell thicknesses found in those studies were significantly different from those found here， which makescomparisons rather difficult. 久/nm Figure 8. UV/Vis absorption spectra of laser-generated iron-gold nanoparticles in water， acetone， andMMA. Figure 9. (a) Magnetic hysteresis loop oflaser-generated FeAu nanoparticles in acetone at T=300K.(b)Normalized experimental data from (a) andLangevin simulations for different Fe core diameters at T=300 K.Note that different field ranges are displayed. In addition to the optical properties of the Fe@Au nanoparticles， the magnetic properties of the bimetallicnanoparticles were also characterized. The magnetic measurements of nanoparticles formed by laser ablation inacetone are depicted in Fig. 9. A small open hysteresis with a coercive field of 4 mT was observed (Fig. 9a). Thesaturation magnetization of 10.3 emu/g was reached at field strengths exceeding 500 mT (Fig. 9b).Consideringthe size distribution of the Fe cores， with diameters of 5-75 nm (Fig. 1，Supplementary Fig. S1)， different magneticresponses from (i) superparamagnetic， (ii) single-domain blocked， and (iii) multidomain nanoparticles would beexpected at T=300 K. Assuming simple estimates using bulk values， we expect superparamagnetic nanoparticlesbelow Fe core diameters of 16 nm and multidomain particles above 50 nm68 Thereby， the coercive field is maxi-mized for diameters near the multidomain limit. However， the overall shape of the hysteresis loop in Fig. 9a witha small coercive field， an almost negligible remnant magnetization， and large saturation fields suggests that mostof the magnetic response originates from superparamagnetic particles. Even though it is well known that particleswith Fe at% <25% exhibit paramagnetic properties， this consideration is not relevant in this system as the parti-cles all had iron enriched cores (compare Fig.4). Figure 9b presents the normalized experimental data togetherwith simulated Langevin functions for 5 nm， 7nm， and 10 nm Fe core diameters. An unambiguous demonstrationof the origin of the magnetic features would require detailed temperature-dependent measurements， which arebeyond the scope of the present contribution. Although none of the simulations accurately fit the experimentaldata， it is clear that the dominating magnetic signal arises from small nanoparticles with iron core diameters wellbelow 10 nm. This discrepancy compared to the Fe core size distribution measured using TEM (15nm， Fig. 4d)could be explained by either significantly reduced magnetic anisotropy in the FeAu nanoparticles and a reducedmagnetization of the Fe core (by forming Fe-rich alloys) or by small Fe particles surrounded by Au inside theparticle core. Conclusions In conclusion， the presented work describes a suitable approach for the one-step synthesis of core-shell Fe-Aunanoparticles by pulsed laser ablation in liquids without the use of reducing agents or organic stabilizers. Adetailed elemental analysis revealed that the particle internal phase composition can be controlled by the chem-ical interactions of the particle surface with its environment. This well-established correlation is exploited to control the nanoscopic particle structure by varying the applied solvents. In the case of the FeAu system， organicsolvents able to stabilize gold surfaces are used to produce a gold shell with an iron core. In contrast， aqueousmedia favor the formation of a hydrophilic oxide shell on the particle surface. Solvent-controlled core-shell nan-oparticles are the predominant species formed in both cases. These findings could offer fundamental insightsinto the particle-formation process during PLAL， indicating that solvent-metal/metal oxide interactions are akey factor dominating the internal phase composition of alloy nanoparticles because surface-metal interactionsdrive elemental segregation. In addition to fundamental insights into the particle formation process， this workalso highlights the synthesis of a hybrid material composed of an Fe@Au core-shell structure in acetone. Theseparticles exhibit outstanding oxidation resistance and moderate magnetic (saturation magnetization 10.3 emu/g)and plasmonic properties. Although these features may seem inferior relative to， e.g.， the magnetic properties ofpure Fe or FeO4 nanoparticles (saturation magnetization： 218 emu/g)9or the plasmonic properties of pure Aunanoparticles， the combination of these features into one entity could open up novel avenues for applications suchas biomedical dual imaging. In addition， the excellent oxidation resistance of the material could be beneficial dur-ing long-term applications， where high signal stability is paramount. Moreover， the dense gold shell covering theiron core can be easily conjugated by functional biomolecules via thiol linkers11，13. 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Acknowledgements We thank Prof. Farle and Prof. Lorke (both University of Duisburg-Essen) for helpful discussions. PhilippWagener thanks the German Ministry of Education and Research(BMBF) for generous funding (NanoMatFutur，FKZ 03X5523). The authors also thank the German Research Foundation (DFG) for funding of the FeAu researchproject (BA 3580/18-1，KI 1263/15-1). The authors also thank the Karlsruhe Nano Micro Facility for ElectronMicroscopy and Spectroscopy. Author Contributions P.W. design of study， data interpretation， wrote the first draft of the manuscript. J.J. conducted particle synthesis，performed analytical zeta potential， DLS， UV-Vis measurements as well as basic electron microscopy studies.He， furthermore， contributed to data interpretation and designed all figures for the manuscript. C.R. wrote thefinal version of the manuscript and contributed to data interpretation. V.S.K.C. conducted electron microscopicstudies and interpretations and wrote parts of the manuscript. C.T. performed analysis of magnetic properties.U.W. interpretation of studies on magnetic properties， wrote the corresponding part of the manuscript. M.B.performed the focused ion beam studies and interpreted the results. L.K. supervised the electron microscopystudies， aided in result interpretation and discussion， and wrote parts of the manuscript. S.B. design of study， datainterpretation， supervised the whole study， and wrote parts of the manuscript. Additional Information Supplementary information accompanies this paper at http：//www.nature.com/srep Competing financial interests： The authors declare no competing financial interests. How to cite this article： Wagener， P. et al. Solvent-surface interactions control the phase structure in laser-generated iron-gold core-shell nanoparticles. Sci. Rep. 6，23352； doi： 10.1038/srep23352(2016). This work is licensed under a Creative Commons Attribution 4.0 International License. The imagesor other third party material in this article are included in the article’s Creative Commons license，unless indicated otherwise in the credit line； if the material is not included under the Creative Commons license，users will need to obtain permission from the license holder to reproduce the material. To view a copy of thislicense， visit http：//creativecommons.org/licenses/by/4.0/ SCIENTIFICREPORTS|：DOI：srep This work highlights a strategy for the one-step synthesis of FeAu nanoparticles by the pulsed laserablation of alloy targets in the presence of different solvents. This method allows particle generationwithout the use of additional chemicals; hence, solvent-metal interactions could be studied withoutcross effects from organic surface ligands. A detailed analysis of generated particles via transmissionelectron microscopy in combination with EDX elemental mapping could conclusively verify that thenature of the used solvent governs the internal phase structure of the formed nanoparticles. In thepresence of acetone or methyl methacrylate, a gold shell covering a non-oxidized iron core was formed,whereas in aqueous media, an Au core with an Fe3O4 shell was generated. This core-shell morphologywas the predominant species found in >90% of the examined nanoparticles. These findings indicatethat fundamental chemical interactions between the nanoparticle surface and the solvent significantlycontribute to phase segregation and elemental distribution in FeAu nanoparticles. A consecutiveanalysis of resulting Fe@Au core-shell nanoparticles revealed outstanding oxidation resistanceand fair magnetic and optical properties. In particular, the combination of these features with highstability magnetism and plasmonics may create new opportunities for this hybrid material in imagingapplications.