Cite this: RSC Advances, 2013, 3,17954

Thermally annealed Ag nanoparticles on anodizedaluminium oxide for SERS sensing

Received 20th July 2013,Accepted 31st July 2013

DOI: 10.1039/c3ra43808b

www.rsc.org/advances

Polina Pinkhasova,a Hui Chen,a M. W. G. M. (Tiny) Verhoeven,b Svetlana Sukhishvilic

and Henry Du*a

In this paper we report a study of novel thermally stable surface enhanced Raman scattering (SERS)

substrates consisting of Ag nanoparticles immobilized on an anodized aluminum oxide (AAO) support.

The morphological and chemical characteristics and the SERS activity of the Ag nanoparticles before and

after the thermal treatment were evaluated using SEM, XPS, UV-vis and Raman spectroscopy. Our results

show that the nanoporous surface of AAO significantly hinders the fusion of Ag nanoparticles to single

large particles at up to 400 uC, preserving high SERS enhancement. XPS and SERS results indicate that

exposure to high temperatures efficiently ‘cleanses’ the surface from citrate remnants, opening more

binding sites for analytes on Ag nanoparticles. In addition, thermal decomposition of silver oxide occurs at

400 uC, ensuring a pure metallic surface and further enhancing SERS activity.

Introduction

The capability of Ag plasmonic nanostructure for SERS-basedchemical and biological sensing and measurements is broadlyrecognized. The advantage of SERS combines the capability ofmolecular fingerprinting and label-free detection of chemicaland biological species with ultrahigh, and in some casessingle-molecule sensitivity.1,2 Significant advances have beenmade in extensively exploring plasmonic properties of Agnanoparticles (Ag NPs) for SERS at room temperature.However, robust as they are, Ag NPs have limited utility for ahost of other sensing applications at elevated temperaturesdue to their tendency to coalesce or to Ostwald ripen, whichdiminishes SERS enhancement. Examples include the probingof catalytic reactions on a hot support3,4 and the monitoring ofcombustion products in gas turbines5,6 and coal gasifiers.7

The perceived susceptibility of Ag to oxidation further adds tothe challenge of the utility of Ag NPs for high-temperatureSERS.

Numerous efforts have been made to stabilize Ag nanos-tructures at high temperatures. Van Duyne et al. demonstratedthat by applying a highly refractory Al2O3 thin film via atomiclayer deposition, the triangular Ag nanostructure fabricated by

nanosphere lithography retained its shape at up to 500 uC ininert environment.8 The unprotected counterpart started tolose its pointed features and eventually became rounded dueto thermal diffusion of Ag atoms to achieve a configuration ofminimal surface free energy. Dai and his co-workers followed asimilar protective coating strategy and showed that Ag films9,10

and nanowires11,12 with a thin Al2O3 layer exhibited goodmorphological stability and SERS enhancement factor (EF) ofabout 5.5 6 104 at 400 uC. In contrast, the coating-free Agfilms and nanowires were deprived of their nanoscopicfeatures and rendered completely inactive in SERS after thesame thermal treatment.

Al2O3 thin films have been proven effective in morpholo-gical stabilization and SERS preservation of Ag nanostructuresat high temperatures. They have an unintended adverse effecton SERS enhancement for two reasons, however. One is theprevention of the adsorption of analyte molecules directly atthe Ag surface, rendering the chemical enhancement mechan-ism, which is responsible for 102 in overall SERS EF,inoperative. The other is the separation of the analytemolecules by a distance equivalent to the coating thickness,reducing the electromagnetic enhancement, which is theprincipal source of EF, due to rapid decay of the electro-magnetic field from the Ag surface. It has been wellestablished that the highest SERS enhancement occurs whenan analyte molecule is situated at the surface of the noblemetal nanostructure where both the chemical and electro-magnetic enhancements are in full force.13,14 It is thereforeadvantageous to develop a SERS platform that maintains themorphological stability of the Ag NPs at high temperatureswithout surface passivation or encapsulation and, that suchplatform is sensitive and reproducible.

aDepartment of Chemical Engineering and Materials Science, Stevens Institute of

Technology, Hoboken, NJ 07030, USA. E-mail: hdu@stevens.edu; Fax: (201)

216-8306; Tel: (201) 216-5262bSchuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513,

5600 MB Eindhoven, The Netherlands. E-mail: M.W.G.M.Verhoeven@tue.nl;

Fax: (040) 247-3481; Tel: (040) 247-3081cDepartment of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens

Institute of Technology, Hoboken, NJ 07030, USA. E-mail: ssukhish@stevens.edu;

Fax: (201) 216-8240; Tel: (201) 216-5544

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Here, we use another approach to generate substrates withinhibited coalescence of Ag NPs at high temperatures for highsensitivity SERS sensing. Our approach is based on adsorptionof colloidal Ag NPs at the surface of anodized aluminum oxide(AAO) membrane. The approach does not require pre-coatingAg nanostructures with thin layers of Al2O3 for stabilization athigh temperatures, and provides strong SERS activity enabledby exposure of a pristine silver surface to an analyte. The freshmetallic surface is a result of decomposition of native silveroxide as well as surface ‘cleansing’ of adsorbed organicmolecules during heat treatment of the substrate at elevatedtemperatures.

The use of porous alumina membranes as SERS substrateshas been successfully employed in the SERS community bytaking advantage of its 3D porous nanostructure.15,16 Forexample, Tsukruk and Ko were able to gain high EFs throughoptical waveguide effect of vertical alumina-pore arrays withexcited evanescent electrical field by decorating nanocanalarray of AAO with Au nanoparticles.17 Moskovits and hiscoworkers fabricated Ag nanowires on porous aluminumoxide, and were able to demonstrate that plasmonic origin ofthe most intense SERS signals such as those responsible forsingle molecule SERS is highly dependent on system’snanogeometry.18

While the use of porous alumina membranes as SERSsubstrates has been employed, these approaches usuallyinvolved electrodeposition,15,18 sputtering19,20 or in situgrowth21 of silver nanostructures smaller or equal to the poresize within nanoporous substrates. Except for a recent reportby Takahashi and coworkers on an increased stability ofelectrodeposited Au and Ag nanostructures deposited on ITOelectrode using Al2O3 as a nanomask,22 high-temperaturestability of these nanopore-templated silver nanostructuresremain unstudied. Moreover, the effect of high-temperaturepre-annealing of AAO-deposited noble metal clusters on theirSERS performance has not been explored. Note that theapplications of other techniques, such as the nanoclusterdeposition technique, to create Ag nanoclusters on smoothrather than nanostructured substrates, lead to heterogeneous,large-size Ag surface clusters after annealing at 300 uC, and theresultant low SERS activity of the substrates.23

In contrast to prior studies, we use colloidal Ag NPs forimmobilization at the surface of the AAO membrane. The highperformance of these substrates at high temperature stemsfrom the fact that anchoring of larger Ag NPs on the porousAAO surface of smaller pore sizes serves as a physicalhindrance to coalescence and fusion of Ag NPs in closeproximity at high temperatures. We report that substrate nano-topography significantly hinders the rapid fusion of Ag NPs onAAO, as often encountered on planar Si, in the temperaturerange up to 400 uC. Importantly, we show that the combinedeffect of morphological stabilization of Ag NPs on AAOmembrane and decomposition of silver oxide to pristine metalduring high temperature annealing results in an enhancementof the SERS activity of the substrate. These results illustrate apromising potential of uncoated Ag plasmonic nanostructures

on AAO platform for SERS sensing and measurements at hightemperatures.

Experimental

Materials

All chemical reagents were purchased from the indicatedsuppliers and used without further purification: silver nitrate(ultrapure grade, Acros), sodium citrate dehydrate (enzymegrade, Fisher), rhodamine 6G (R6G; Aldrich), poly(allylaminehyrdochloride) (PAH; weight-average molecular weight of15 000 g mol21, Aldrich), high purity aluminum foil (0.1 mmthick, 99.99%, Alfa Aesar), rectangular aluminum bar (alloy6063, rounded edge, 1/8’’ thick, McMaster-Carr), ethanol(anhydrous, 99.8%, Aldrich), perchloric acid (60–62%, Acros),phosphoric acid (85.3%, Acros). Water used for experimentswas filtered with Barnstead ion-exchange columns and thenfurther purified by passage through Millipore (Milli-Q)columns. All glassware and substrates were cleaned innochromix (Godax Laboratories, Inc.) solution in concentratedsulfuric acid overnight, followed by thorough rinsing withMilli-Q water.

Synthesis and characterization of Ag NPs

Ag NPs were synthesized by a modified Lee and Meiselmethod.24 0.8 mL of 1% sodium citrate was added in a drop-wise fashion to 40 mL solution of 1 mM AgNO3. The mixturewas stirred for 1 min and transferred into a UV chamber (UVflood curing system, cure zone 2; CON-TROL-CURE, Chicago,IL). UV irradiation has a heating effect on an otherwise room-temperature synthesis process. To prevent excessive tempera-ture increase, a water bath was used to keep the temperature ofthe solution mixture below 50 uC to ensure quality synthesis ofmonodispersed Ag NPs. The reaction mixture was kept underUV irradiation with continuous stirring for 4 h. The resultantparticles had a f-potential of 235 mV with an average particlesize of 40 ¡ 5 nm and a plasmon resonance at 408 nm. Thesize and distribution, surface charge, and plasmon resonanceof the Ag NPs were determined by transmission electronmicroscopy (TEM, Philips CM20), scanning electron micro-scopy (SEM, Auriga Zeiss), dynamic light scattering (ZetasizerNano Series, Malvern Instruments), and UV-visible absorptionspectroscopy (Synergy HT multidetection microplate reader,BioTek Instruments).

Preparation of AAO

AAO was prepared by anodization of high purity aluminumfoil. Aluminum foil was first cleaned by sonication in acetoneand then in Milli-Q water for 10 min. It was electropolished inthe mixture of perchloric acid and ethanol (with volume ratioof 1 : 4), under 10 V for 2 min at room temperature.Anodization was subsequently conducted in 0.3 M phosphoricacid under 20 V for 6 min at 0 uC. During anodization,electrolyte solution was vigorously stirred. The applied voltageand current were monitored by Keithley 2420 source inconjunction with LabVIEW program. The morphologicalcharacteristics of AAO were investigated using SEM. The

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average pore size was 20 ¡ 5 nm with a pore depth of about 60nm. The pore depth was determined from SEM images offractured membranes under the viewing angle of 52u, whichwas accounted for in estimating the thickness.

Preparation of SERS platforms, XPS UV-vis samples

SERS platforms were prepared by immobilizing Ag NPs on AAOor on single crystal Si covered by a thin native oxide. AAO andSi were first immersed in 0.2 mg mL21 PAH at pH 9 for 20 min.They were subsequently rinsed with Milli-Q water at pH 4.5 toremove any free or loosely bound PAH. Ag NPs were attachedto the PAH-modified surface using a solution of 1016 particlesml21 at pH 5.5 for 4 h. Ag NPs were immobilized throughelectrostatic interactions of positively charged PAH surfaceand negatively charged Ag NPs. Porous AAO did not play a rolein particle immobilization, only positively charged aminogroup of PAH did. We note that there is excellent reproduci-bility in terms of the distribution and coverage density ofimmobilized Ag NPs under any given condition as ascertainedby SEM. The coverage density was determined using ImageJsoftware of 7 images for each substrate. AAO and Si withimmobilized Ag NPs were thoroughly rinsed with Milli-Q waterand were used immediately for SERS measurements or heattreatment. The SERS platforms were treated at 200 uC, 300 uC,400 uC and 600 uC in a quartz tube furnace (MellenMicrotherm MT) for 20 min in laboratory ambient. The heat-treated platforms were assessed also for their SERS activity aswell as the morphological stability using SEM. Ag NPsimmobilized on Si substrates with or without heat treatmentwere used for XPS studies.

UV-vis samples were prepared in the exact same fashion asSERS substrates described above, except a glass slide was usedas a support. Ag NPs immobilized on a glass slide wereannealed in a similar fashion as described above. Once cooledto room temperature glass slides were placed in a quartscuvette filled with Milli-Q water and UV-visible absorptionspectroscopy was measured (Lambda 25, Perkin Elmer). All thedata was processed using Origin 8.0 software.

SERS and XPS measurements

SERS measurements were conducted using 632.8 nm excita-tion wavelength using a custom-built Raman spectrometer.25

Analytical EFs were calculated using EF = (ISERS/CSERS)/(INR/CNR), where ISERS and INR are integrated intensities of acharacteristic band from SERS and normal Raman, and CSERS

and CNR are concentrations of analytes used in SERS andnormal Raman experiments, respectively. Both intensities ofnormal Raman and SERS were collected under the sameconditions (same power, wavelength, and acquisition time),and the volume of the analyte solution was kept constant at100 mL. The intensity of R6G peak at 1516 cm21 was used tocalculate the EFs. The intensity of this peak was normalized tothe appropriate concentration of the analyte present on thesubstrate. To ensure statistical significance, SERS measure-ments and calculated EFs were done on two identicallyprepared substrates of the same nanostructure type. Threemeasurements were done on each of those substrates andspot-to-spot variation was less than 15%. SERS data wasprocessed using Origin 8.0 software. The XPS measurements

were carried out with Thermo Scientific K-Alpha, equippedwith a monochromatic small spot X-ray source and a 180O

double focusing hemispherical analyzer with a 128-channeldetector. Spectra were obtained using an aluminum anode (AlKa = 1486.6 eV) operating at 72 W and a pot size of 400 mm.Survey scans were measured at a constant pass energy of 200eV and region scans at 50 eV. The background pressure was 26 1029 mbar and during measurements 4 6 1027 mbar Argonbecause of the charge compensation dual beam source.Binding energies were calibrated using the Si 2p peak ofamorphous silica at 103.3 eV.

Results and discussion

Citrate reduced Ag NPs were immobilized on nanoporous AAOand on smooth Si substrate. AAO was prepared usingprocedures described in the experimental section. Thestructural characteristics of AAO as determined by SEM areshown in Fig. 1. In brief, it has an average pore size of 20 ¡ 5nm and an overall thickness of 60 nm. Negatively charged AgNPs with an average diameter of 40 ¡ 5 nm were immobilizedon AAO and Si through electrostatic interactions using apolymer mediated approach. Surfaces of both AAO and Sisubstrates were first modified with a monolayer of adsorbedpositively charged poly(allylamine hydrochloride) (PAH) with

Fig. 1 SEM images of anodic aluminum oxide (AAO) with an average pore sizeof 20 ¡ 7 nm (a) and an overall thickness of 60 nm (actual AAO thickness of48.55 nm/the viewing angle of sin 52u = 60 nm) (b).

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9 Å in dry thickness, as measured by ellipsometry with Sisubstrates. Ag NPs were then immobilized through electro-static interactions of positively charged amino groups on PAHand negatively charged, citrate-reduced Ag NPs. SEM images ofimmobilized Ag NPs are shown in Fig. 2A (a) for AAO andFig. 2B (a) for Si. The coverage density of Ag NPs on both AAOand Si was y250 ¡ 10 particles mm22, estimated from SEMimages using ImageJ.

We examined the morphological stability of Ag NPs on AAOand Si at 200 uC, 300 uC and 400 uC. There is virtually nochange in the overall morphology, including particle size anddistribution, for Ag NPs on AAO after heat treatment at 200 uC,compared to the untreated counterpart, as illustrated in theSEM micrographs in Fig. 2A (a and b). As the temperature wasraised to 300 uC, Ag NPs maintained their general morpholo-gical features except for those situated right next to each other

Fig. 2 SEM images of Ag NPs immobilized on AAO (A) and Si (B), and heat treated for 20 min at 25 uC (a), 200 uC (b), 300 uC (c) and 400 uC (d) in ambient air. Thecoverage density of Ag NPs on both AAO and Si at 25 uC is y250 ¡ 10 particles mm22, estimated from SEM images using ImageJ software.

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that began to fuse together at the junctions, as seen in Fig. 2A(c). Same can be said for the Ag NPs treated at 400 uC but withsubstantial coalescence of the particles in contact though theoriginal particles remained in their initial sites as shown inFig. 2A (d). Ag NPs on Si exhibited substantial morphologicalchanges after the heat treatment starting at 200 uC. A commonfeature is that all Ag NPs in close proximity fuse to form single,large particles. The size expansion of the fused particles led tofurther annexation of neighboring particles on Si. Particleenlargements intensified with increased temperature, result-ing in increased average particle size and reduced particlecoverage density as illustrated in Fig. 2B (a–d). The strikingdifference in the thermal stability of Ag NPs on AAO and Si is aclear demonstration that the nanoporosity of AAO functionedas a physical hindrance to the fusion of Ag NPs up to 400 uC.

The morphological changes of Ag NPs on Si induced byannealing are consistent with the UV-vis absorption character-istics of Ag NPs on smooth glass substrate after heat treatmentunder identical conditions. Fig. 3 (a–d) shows the evolution, asa function of annealing temperature, of the absorption spectraof the corresponding samples. The spectrum for the unan-nealed sample consists of two bands: the main band at 405 nmis associated with the plasmon resonance of isolated Ag NPs aswell as the transverse plasmon mode of Ag NP clusters; theweaker band centered around 695 nm is a contribution fromthe longitudinal mode of plasmon coupling of these clus-ters.26–28 After heat treatment at 200 uC the main plasmonband starts to broaden and red shifts to 410 nm, while theweaker band drastically blue shifts to 530 nm due to fusion ofAg NP clusters. After exposure to 300 uC only one plasmonband remains, that is further red shifted to 424 nm due toincrease in size,29 and the contribution from longitudinalmode completely disappears, suggesting that only sphericalparticle are present. Further annealing at 400 uC splits thisplasmon band into two components at 324 nm and 468 nm,indicating a bimodal distribution of spherical particles, as canbe seen from SEM images in Fig. 2 (B-d).

How stable Ag NPs are, thermally and chemically, has aprofound influence on the SERS enhancement by virtue of thesensitivity of the localized surface plasmon resonance (LSPR)to their size, shape, and surface chemistry.8 We used R6G as amodel analyte to carry out ambient-temperature SERS mea-surements using Ag NPs on AAO and Si with similar coveragedensity before and after the various heat treatments. Shown inFig. 4A and 4B are respective SERS spectra of 50 ppb R6G withAg NPs on AAO and on Si with temperature as a parameter.Note that in both substrate types, the Raman intensityunderwent a striking and rapid reduction upon annealing at200 uC, as compared to unannealed samples. While thedifference is relatively minor for annealed samples at 200 uCand 300 uC, there is a marked increase in the Raman intensityafter annealing at 400 uC for both substrates. Fig. 4C showsanalytical EFs calculated using a band at 1516 cm21 of R6G forboth substrates as a function of annealing temperature.According to the figure, there is an interesting trend in theEF versus temperature correlation for Ag NPs on both AAO andSi. The EF experienced a substantial drop, more prominentlyin the case of Si substrate, after annealing at 200 uC. In thecase of AAO, the EF continued its downward trend at 300 uCbut recovered significantly at 400 uC to near the EF ofunannealed sample. In contrast, with Si, the EF experiencedonly slight and gradual recovery at both 300 uC and 400 uC butremained far below the EF of the annealed sample. Ag NPs onAAO consistently exhibited markedly higher EFs than on Siunder all annealing conditions.

While the temperature-dependent morphological changescan give us significant clue to the observed correlationbetween EF and temperature, the surface chemistry of AgNPs and its temperature dependence also needs to beestablished to provide clear insights into the complex relation-ship. To this end, we conducted SERS measurements with AgNPs on AAO and Si in the absence of any analytes before andafter annealing treatments. Prior to annealing, Ag NPs on AAOand Si exhibited a rich Raman background as shown in Fig. 5(A-a) and (B-a). This result is expected due to adsorbed citrateremnants inherent to the particle synthesis. The Ramanbackground associated with citrate remnants was greatlyreduced upon annealing at 200 uC and completely removedat 400 uC in both cases, indicating their thermal desorption,opening more sites for the adsorption of analytes.

XPS studies yielded further information on the intricatechanges in the surface chemistry of Ag NPs. Fig. 5C shows highresolution Ag 3d photoelectron spectra of Ag NPs on Si beforeand after annealing in laboratory ambient. The spectrum foras-prepared Ag NPs has its main Ag 3d5/2 peak situated at 367.5eV with a shoulder at 367.3 eV (Fig. 5C (a)) according to peakdeconvolution analysis, characteristic of Ag2O and AgOrespectively. Upon closer inspection the asymmetry of thepeak is more evident and this result is consistent with the factthat Ag is prone to oxidation, though generally slight. Thisshoulder disappears and the Ag 3d photoelectron spectra forAg NPs becomes symmetric with the Ag 3d5/2 peak centered at367.7 eV after annealing at 200 uC and 300 uC as illustrated in

Fig. 3 UV-vis spectra of Ag NPs immobilized on a glass substrate and annealedfor 20 min at 25 uC (a), 200 uC (b), 300 uC (c), and 400 uC (d).

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Fig. 5C (b and c), suggesting the decomposition of AgO and asurface rich in Ag2O. The Ag 3d5/2 shifted significantly to 368.2eV after annealing at 400 uC as shown in Fig. 5C (d), indicatingthe decomposition of Ag2O to Ag and hence the metallicnature of the surface of Ag NPs. Our XPS results are consistent

with documented studies of related systems.30,31 For example,AgO was shown to be thermally reduced to pure metallic silverfollowing two irreversible steps: in the temperature region of100 uC to 200 uC, it thermally decomposes to Ag2O, followed bycomplete decomposition to metallic Ag at 400 uC.32 Inaddition, thermal decomposition of Ag2O thin film inoxygen-rich environment31 and AgO powder in ultra-highvacuum30 both to metallic Ag has been observed at around 400

Fig. 5 SERS spectra of citrate-reduced Ag NPs immobilized on (A) AAO and (B) Sicollected at room temperature upon heat treatment at 25 uC (a), 200 uC (b), 300uC (c), and 400 uC (d). Spectra were acquired at 633 nm (4 mW, 20 s). (C) XPS ofAg NPs on Si after heat treatment at 25 uC (a), 200 uC (b), 300 uC (c) and 400 uC(d).

Fig. 4 SERS spectra of R6G obtained at room temperature using Ag NPsimmobilized on AAO (A) and Si (B). Substrates were heat treated for 20 min at(a) 25 uC, (b) 200 uC, (c) 300 uC, and (d) 400 uC. The characteristic bands for R6Gat y611 cm21, y777 cm21 correspond to d(C–C–C) ring in-plane and out-of-plane respectively, y1368 cm21, y1516 cm21, and y1655 cm21 correspond ton(C–C) vibration in the aromatic ring. Spectra were acquired at 633 nm (4 mW,20 s). (C) Summary of calculated SERS EFs for R6G using the band at y1516cm21 for substrates with immobilized Ag NPs on AAO (in blue) and Si (in red).

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uC. Importantly, here we correlate these temperature inducedchanges in oxidation of Ag NPs with their SERS activity.

Taking the changes in morphology and surface chemistryof Ag NPs as a whole and considering their interplay in SERSenhancement, we offer the following interpretation on thecorrelation between the analytical EFs and annealing tem-perature for Ag NPs on AAO and Si shown in Fig. 4C. Ag NPsimmobilized on both substrates are mostly discretely dis-tributed, there exists a significantly portion of dimers and eventrimers in direct contact or separated by nm gap (Fig. 2A (a)and 2B (a)). These clusters are known to form hot spots thatgive rise to very high SERS sensitivities. Upon annealing at 200uC and due to high surface atom mobility, necking takes placefor Ag NPs on AAO and fusion into larger individual sphericalparticles occurs for Ag NPs on Si (Fig. 2A (b) and 2B (b)),leading to reduction in hot spots on AAO and elimination onSi. This process is responsible for the rapid decrease in the EFsin both cases although thermal desorption of citrate remnantshas the benefit of clean SERS background as well as increasedbinding sites for analytes. Nanoporous AAO clearly played asignificant role in preventing fusion of Ag NPs in closeproximity into single larger particles as seen in the case ofsmooth Si substrate. Thus, some hot spots still persisted at theparticle junctions despite significant necking, resulting in aconsiderably higher EF on AAO than on Si. As temperature isincreased to 300 uC, necking continued for the Ag NPs incontact on AAO, leading to further reduction in the hot spotsand continued decline in EF. In contrast, the EF increasedslightly for Ag NPs on Si due to increased availability of surface

binding sites as hot spots were no longer a factor at thistemperature.

Importantly, at 400 uC, the EF of Ag NPs on AAO underwentan increase to a value comparable to that before anneal. Thisincrease can be attributed to the metallic Ag surface as a resultof the decomposition of Ag2O to Ag. We have previously shownthat oxidation of Ag in a form of only a submonolayersignificantly decreases the EF due to reduction in binding sitesas well as retardation of the electronic enhancement.33 Thereare signs of fusion for Ag NPs in contact on AAO at 400 uC dueto significantly highly atomic mobility. However, the originalnanoparticle features remain identifiable, in sharp contrast tothe continued and accelerated coalescence of Ag NPs on Si,which lead to the formation of Ag particles as large as 300 nmin diameter. The substantial increase in particle size on Sireduces greatly the SERS activity. The adverse effect of sizeincrease counters the beneficial effect of free metal surface,yielding a far smaller increase in the EF of Ag NPs on Si afterannealing at 400 uC, compared to 300 uC and far lower than theEF before annealing.

Finally, we explored the robustness of Ag NPs on AAO in acyclic experiment: repeated annealing at 400 uC and SERSmeasurements using 50 ppb R6G, in conjunction with SEMcharacterization of the morphology after each annealingtreatment. Shown in Fig. 6 are the SERS spectra and theSEM images for each of the three cyclic studies. Clearly there isno degradation in SERS enhancement and in nanoparticlemorphologies after each treatment step. The exceptionalreproducibility of SERS performance of these substrates as

Fig. 6 SERS spectra of 50 ppb R6G obtained at room temperature using Ag NPs immobilized on AAO and heat treated for 20 min at 400 uC for three cycles. Spectrawere acquired at 633 nm (4 mW, 20 s). SEM micrographs illustrate the morphology of Ag NPs after each cycle of heat treatment at 400 uC.

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annealing at 400 uC effectively cleanses any pre-adsorbedanalytes, suggesting excellent potential of Ag NPs on AAO forreuse at this temperature.

Conclusions

We have shown that Ag NPs immobilized on AAO haveexcellent potential for SERS sensing and measurements atelevated temperatures. Although the EF decreases uponannealing at 200 uC and 300 uC due to necking of Ag NPs incontact thus reducing hot spots, the morphological stabilityrendered by AAO and the decomposition of silver oxide on AgNPs to fresh Ag at 400 uC combine to recover the EF almost tothe level prior to the annealing treatment. We have demon-strated the feasibility of reuse of the SERS substrate at 400 uC.The nanoporous AAO clearly has played a major role as ahindrance to fusion of Ag NPs in close contact to single largeand less SERS-active particles at up to 400 uC. In contrast, AgNPs in close contact on Si continue the fusion that leads tosingle particles of growing dimension and annexation ofneighboring particles in the temperature range investigated,resulting in substantial reduction in EF. The demonstratedmorphological stability of Ag NPs on AAO is a strongindication of their potential for a multitude of applicationssuch as process monitoring in energy generation and produc-tion, molecular adsorption and desorption during high-temperature catalysis involving precious metals.

Acknowledgements

This work was partially sponsored by the US National ScienceFoundation under grants OISE-1157600 and ECCS-1325367.

Notes and references

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