Carbon-Dot/Silver-Nanoparticle Flexible SERS-Active FilmsSusanta Kumar Bhunia,† Leila Zeiri,‡ Joydeb Manna,†,§ Sukhendu Nandi,†,∥ and Raz Jelinek*,†,‡

†Department of Chemistry, Ben Gurion University of the Negev, Beer Sheva 84105, Israel‡Ilse Katz Institute for Nanotechnology, Ben Gurion University of the Negev, Beer Sheva 84105, Israel

*S Supporting Information

ABSTRACT: Development of effective platforms for surfaceenhanced Raman scattering (SERS) sensing has mostly focusedon fabrication of colloidal metal surfaces and tuning of their surfacemorphologies, designed to create “hot spots” in which plasmonicfields yield enhanced SERS signals. We fabricated distinctive SERS-active flexible films comprising polydimethylsiloxane (PDMS)embedding carbon dots (C-dots) and coated with silver nano-particles (Ag NPs). We show that the polymer-associated Ag NPsand C-dots intimately affected the physical properties of eachother. In particular, the C-dot-Ag-NP-polymer films exhibitedSERS properties upon deposition of versatile targets, bothconventional SERS-active dyes as well as bacterial samples. Weshow that the SERS response was correlated to the formation C-dots within the polymer film and the physical proximity between the C-dots and Ag NPs, indicating that coupling between theplasmonic fields of the Ag NPs and C-dots’ excitons constituted a prominent factor in the SERS properties.

KEYWORDS: SERS, carbon dots, Ag nanoparticles, plasmon, bacterial sensors

■ INTRODUCTION

Metal surfaces, particularly gold and silver, constitute substratesfor surface enhanced Raman scattering (SERS), a powerfulsensing technology.1 The fundamental determinants of SERSactivity are primarily traced to the morphology of the metalsurface, specifically the presence of nanostructured plasmonfields, especially around rough surfaces and sharp edges ofmetal colloids.2 To some extent, SERS phenomena were alsoaffected by plasmon couplings and energy transfer processesinvolving metal or semiconducting substances.3 However,despite the considerable interest and vast potential applicationsof SERS-based sensors, there is still limited understanding andinsufficient control of substrate features responsible for SERSsignals. Most efforts toward forming SERS-active surfaces havefocused upon modulating the nanostructural properties ofmetal surfaces aimed at generating SERS “hot spots”.1,2,4−6

Some studies have suggested that fabrication of hybrid surfacescomprising metal NPs and other nanostructures augmentedcharge-transfer processes contributing to more pronouncedSERS signals.7,8

Carbon quantum dots (denoted “C-dots”) are promisingnew carbon nanoparticles exhibiting interesting photophysicaland electronic properties. C-dots encompass small fluorescentNPs (2−20 nm diameters) comprising graphitic coressurrounded by varied surface functional units.9,10 Surfaceproperties of C-dots have been linked to their uniqueexcitation-dependent emission properties.11,12 C-dots haveattracted significant interest as a useful analytical and sensingplatform due to their broad color range, fluorescence

brightness, stability, biocompatibility and low cytotoxicity,inexpensive and readily available reagents, and simple synthesisprocedures.13−18 Few studies have reported conjugation of C-dots with plasmonic metal NPs and the use of such systems inphotonic and sensing applications.19−23 Notably, C-dots insome of those coupled systems served also as reducing agentsfor generation of the metal NPs, exploiting electron donatinggroups at the C-dots’ surfaces.19−21 While in some instances,the C-dots modulated the optical and spectroscopic propertiesof the noble metal NPs localized in close proximity, directevidence for coupling between the plasmon properties of themetal NPs and C-dots’ excitons is still lacking. Furthermore, theC-dot/noble metal NP configurations reported thus fargenerally do not lend themselves to broad-based practicalapplications.Here we describe construction of SERS-active flexible

polydimethylsiloxane (PDMS) films embedding C-dots andsurface-attached silver NPs. PDMS has been widely used as atransparent, flexible polymer matrix for varied molecular andnanoparticle guest species.9,24,25 Recent studies have employedPDMS as a host matrix for photonic C-dot devices.9 The simplesynthesis scheme developed here utilized an ascorbic acidderivative as both the carbonaceous building block for C-dotformation, as well as anchoring and reducing agent forgenerating surface-attached Ag NPs. The resultant flexible

Received: August 30, 2016Accepted: September 1, 2016Published: September 1, 2016

Research Article

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© 2016 American Chemical Society 25637 DOI: 10.1021/acsami.6b10945ACS Appl. Mater. Interfaces 2016, 8, 25637−25643

hybrid C-dot-Ag films displayed remarkable SERS properties,giving rise to intense signals both from conventional SERSprobes as well as bacterial cells. Notably, the experimental datademonstrate that the SERS signals were specifically related tothe presence of C-dots in proximity to the PDMS-bound AgNPs, thus likely enabling energy and charge transfer processesbetween the two NP species.

■ EXPERIMENTAL SECTIONMaterials. L-(+)-ascorbic acid, silver acetate, oleylamine, sodium

sulfate, 4-amino thio phenol (4-ATP), and pyridine were purchasedfrom Sigma-Aldrich, U.S.A. L-(+)-Tartaric acid was purchased fromAlfa-Aesar, England. Lauroyl chloride was purchased from TCI, Japan.Chloroform, toluene, and n-hexane were bought from Daejungchemicals, Korea. Tetrahydrofuran was purchased from AcrosOrganics, U.S.A. Dimethylformamide (DMF) and acetone werepurchased from Frutarom (Haifa, Israel). Ethyl acetate andconcentrated hydrochloric acid were purchased from Bio-Lab Ltd.(Jerusalem, Israel). Precursors for PDMS film formation (Sylgard 184silicone elastomer base and Sylgard 184 silicone elastomer curingagent) were purchased from Dow Corning Co., U.S.A. All chemicalswere used without further purification.Preparation of PDMS Films Containing the C-Dot Precursor.

The carbon precursor 6-O-(O-O′-dilauroyl-tartaryl)-L-ascorbic acidwas synthesized according to a published report.26 Briefly, 142.3 mL oflauroyl chloride was added to 30 g of finely powdered L-tartaric acid ina round-bottom flask under stirring, and the reaction mixture washeated at 90 °C for 24 h and then cooled to room temperature. Theproduct was precipitated with n-hexane and dried under vacuum toobtain as white powder. To a solution of 20 g L-ascorbic acid in 75 mLanhydrous DMF 11.3 g of the above compound was added. Thereaction mixture was cooled to 0 °C, and 1.83 mL of dry pyridine wasadded. Stirring was continued under argon atmosphere at 0 °C for 30min and then for 3 days at room temperature. After completion of thereaction the mixture was poured into 2 N HCl at 0 °C under vigorousstirring. The reaction mixture was extracted with ethyl acetate, and theorganic fraction was washed 3 times with brine, dried over Na2SO4,filtered, and the solvent was removed under reduced pressure. Theresidue was precipitated twice with n-hexane to obtain as whiteamorphous powder. 50 mg of the ascorbic acid derivative wasdissolved in 200 μL tetrahydrofuran (THF). 800 mg of Sylgard 184silicone elastomer base (PDMS base) was mixed with 80 μL of thesilicone elastomer curing agent in a falcon tube. The C-dot precursorcompound was added into the falcon tube containing the PDMS filmprecursors mixed thoroughly and placed in a vacuum desiccator. The

mixture was subsequently poured into a Petri dish and maintained at70−80 °C for 1 h to make CDs precursor-PDMS film.

Preparation of Ag-Coated C-Dot PDMS Films. 10 mg silveracetate was dissolved in 8 mL toluene with 75 μL oleyl amine. ThePDMS film containing the C-dot precursor prepared according to theprocedure above was dipped into the as-prepared silver salt solutionfor 1 min and washed with toluene several times. The film wassubsequently heated at 125 °C for 2.5 h to form embedded C-dots.

Solution Synthesis of C-Dots. 50 mg of the ascorbic acidderivative and 165 μL deionized water were placed in glass vial andheated at 125 °C for 2.5 h to obtain brown precipitate suggesting theformation of C-dots. The brown precipitate was then redispersed in 5mL of chloroform through vortexing and centrifuged at 14 000 rpm for30 min to remove high-weight precipitate and agglomerated particles.Chloroform was gently evaporated under reduced pressure to obtain abrown solid. The same procedure was repeated with 5 mL acetone,followed by solvent removal under reduced pressure to obtainmonodisperse carbon dots.

Spin-Coating on Polymer Films. 800 mg of Sylgard 184 siliconeelastomer base (PDMS base) was mixed with 80 μL of the siliconeelastomer curing agent in a falcon tube. 30 mg of synthesized C-dotswas dissolved in 200 μL THF and added into the falcon tubecontaining the PDMS film precursors mixed thoroughly and placed ina vacuum desiccator. The mixture was subsequently poured into aPetri dish and maintained at 70−80 °C for 1 h to make C-dots-PDMSfilm. 20 μL of hydroquinone reduced Ag NPs solution was put on C-dots-PDMS film and spin coated at 200 rpm for 20 min to get Ag NPsdeposited on C-dots-PDMS film. A similar spin coating technique wascarried out in the case of poly(vinyl alcohol) (PVA) film.

Characterization. C-dot-Ag-NP-PDMS films were immerged inethyl acetate for extraction of carbon dots and Ag nanoparticles fromthe polymer films. High resolution transmission electron micros-copy (HRTEM) samples were prepared by placing a drop of solutionon a graphene-coated copper grid and observed with a 200 kV JEOLJEM-2100F microscope (Japan). HRTEM images were analyzed byfast Fourier transform (FFT). X-ray photoelectron spectroscopy(XPS) of C-dot-Ag-NP-PDMS film was performed using an X-rayphotoelectron spectrometer ESCALAB 250 ultrahigh vacuum (1 ×10−9 bar) apparatus with an AlKα X-ray source and a monochromator.The X-ray beam size was 500 μm and survey spectra was recorded withpass energy (PE) 150 eV and high energy resolution spectra wererecorded with pass energy (PE) 20 eV. Processing of the XPS resultswas carried out using AVANTGE program. Fluorescence emissionspectra of the C-dot-Ag-NP-PDMS film at different excitationwavelengths were recorded on a FL920 spectrofluorimeter (EdinburghInstruments, U.K.).

Figure 1. Preparation of the C-dot-Ag-NP-PDMS films. A. Synthesis scheme. The ascorbic acid amphiphilic derivative (chemical structure shown onthe left) was first mixed with the PDMS precursors, yielding a PDMS film following heating. Subsequent addition of Ag+ ions and heat-inducedcarbonization generated the C-dot-Ag-NP-PDMS films (yellow dots correspond to C-dots while the red dots indicate Ag nanoparticles). B.Photographs of the PDMS films at different stages of the preparation scheme: i. PDMS film prior to Ag+ addition and carbonization; ii. followingmixing with Ag+ ions−the yellowish color indicates Ag deposition; iii. after thermal treatment (carbonization); iv. Fluorescence image of the film(excitation 560 nm); and v. photograph depicting film flexibility.

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Bacterial Growth. The bacteria used in the studies werePseudomonas aeruginosa PAO1 wild type, Bacillus cereus, and Erwiniaamylovora 238. Bacteria were grown aerobically at 37 °C in a sterilizedsolid LB medium composed of 13.5% yeast extract, 27% peptone, 27%NaCl, and 32.5% agar at pH 7.4. After overnight growth, a colony fromeach bacterial strain was taken and added to 10 mL sterilized LBmedium and incubated at 37 °C. Bacterial growth was monitored atthe desired time points through measuring the concentration of thebacteria by visible spectroscopy (108 CFU mL−1 when optical densityat 600 nm was 1.0). Bacterial cells were separated from the growthmedium through centrifugation followed by washing with water severaltimes and taking suspension in water. To perform the SERSexperiments, a drop (104 CFU mL−1) was placed on film and dried.SERS Measurements. Different concentrations of ethanol solution

of 4-ATP were drop-casted on the C-dot-Ag-NP-PDMS film anddried. SERS spectra were recorded with a Jobin-Yvon LabRam HR 800micro-Raman system equipped with a Synapse CCD detector usingHe−Ne laser at 633 nm and Argon laser at 514.5 nm as excitationsources. The full laser power of 3 mW on the sample was reduced by10−100 using ND filters. The laser was focused with an x100 objectiveto a spot of about 1 μ. The grating used was a 600 g mm−1 and theconfocal hole of 100 μm with exposure times of 10−60 s. Theanalytical enhancement factor (AEF) was calculated according to thefollowing equation:27

=I c

I cAEF

//

SERS SERS

RS RS

where cRS corresponds for concentration of analyte which produces aRaman signal IRS under non-SERS condition. ISERS corresponds forRaman signal of the same analyte on SERS substrate using theexperimental analyte concentration (cSERS).

■ RESULTS AND DISCUSSION

Figure 1 illustrates the synthesis scheme of the C-dot-Ag-NPPDMS films and their optical properties. We started with amixture comprising the PDMS precursors (silicone elastomerbase + curing agent) and an ascorbic acid derivative [6-O-(O-O′-dilauroyl-tartaryl)-L-ascorbic acid]. Following digestion at 60°C a flexible PDMS film was formed, encapsulating the ascorbicacid amphiphiles. The ascorbic acid derivatives had a dual rolein the synthesis, functioning both as the carbon precursors forgeneration of the C-dots, as well as reducing agents of Ag+ ions.Importantly, usage of the amphiphilic ascorbic acid derivativedisplaying covalently attached hydrocarbon chains (Figure 1A)was essential in our experimental strategy, since theencapsulating PDMS matrix is hydrophobic. A recent studysimilarly demonstrated formation of flexible PDMS filmsembedding C-dots through codispersion of carbonaceousprecursors and the elastomer base.9

Following assembly of the PDMS films encapsulating the C-dot precursors, they were placed in silver acetate organicsolution, washed, and heated to 125 °C for 2.5 hyielding thefinal productPDMS film embedding C-dots, with Agparticulates attached onto the film surface (Figure 1A). Figure1B depicts photographs of the hybrid films, highlighting theirfluorescence properties and physical flexibility. The initiallyassembled PDMS film containing the ascorbic acid precursor isshown in Figure 1B,i. After silver deposition, the film had ayellowish brown appearance (Figure 1B,ii), reflecting theaccumulation of metallic silver particles upon the film surface.19

Figure 1B,iii and iv demonstrates that the carbonized hybridfilm displayed a darker brown color and red fluorescence (uponexcitation at 560 nm) confirming formation of C-dots from theascorbic acid derivative embedded within the film. The flexible

and resilient nature of the produced films is shown in Figure1B,v.Figures 2 and 3 present spectroscopic and microscopic

characterization of the C-dot-Ag-PDMS films, particularlyfocusing on the nature of the carbon and silver particlesformed through the synthesis scheme outlined in Figure 1A.The X-ray Photoelectron Spectroscopy (XPS) data in Figure2A illuminate the atomic species present within the films. TheXPS result of the C-dot-Ag-NP-PDMS in Figure 2A,ii showsthe signature peaks of PDMS (Si 2p peaks), as well as confirmsthe formation of metallic silver upon the PDMS film surface(Ag 3d peaks at 368 and 375 eV, Figure 2A).28 Thedeconvoluted C 1s spectrum displays peaks at 284.7 eVcorresponding to sp2 carbon atoms (CC) and 286.0 eVassigned to COH groups, whereas the deconvoluted O 1sspectrum reveals peaks at 532.0 eV for CO and OCOHgroups, and at 533.2 eV corresponding to COH and COC groups. Importantly, the C 1s and O 1s XPS signalsspecifically correspond to atomic species present in C-dots,9

indicating that the carbonization process led to transformationof the PDMS-embedded ascorbic acid amphiphiles into C-dots.Indeed, the XPS of the C-dot-PDMS film (without Ag NP) inFigure 2A,i features C 1s and O 1s XPS signals at the samepositions as the Ag-containing films, indicating formation of C-dots.9

The electron microscopy images in Figure 2B,C, recordedafter dissolution of the PDMS film in ethyl acetate andextraction of the NPs, illuminate the sizes, crystallinity, andcompositions of the carbon and silver nanoparticles producedthrough the synthesis process outlined in Figure 1A. Thetransmission electron microscopy (TEM) image in Figure 2Breveals relatively uniform NPs exhibiting diameters of between2 and 5 nm. The high-resolution TEM (HR-TEM) image inFigure 2B reveals crystalline organizations of both C-dots,displaying lattice spacings of 0.215 nm corresponding to the(110) planes of the graphite core, as well as the Agnanoparticles exhibiting interplanar spacing of 0.235 nmcorresponding to (111) lattice plane of Ag.10,29,30 The fastFourier transform (FFT) diffraction patterns of the twonanoparticles confirm their crystalline lattices (Figure S1,Supporting Information). Scanning-TEM (STEM) analysiscomplemented by energy dispersive spectroscopy (EDS) inFigure 2C (full EDS maps are shown in Figure S2) identifiedNPs comprising silver, corresponding to Ag NPs, while otherparticles were not composed of silver and thus were likely theC-dots.A central question pertaining to the new C-dot-Ag NP-

PDMS system concerns the effects the Ag NPs and C-dotsexert upon the physical properties of each other. Figure 3examines the modulation of the C-dots’ photophysicalproperties following silver deposition. Specifically, Figure 3depicts the excitation-dependent fluorescence emission spectraof C-dot-PDMS films prepared without addition of Ag+ ions(i.e., no Ag NPs deposited, spectra on the left), and the C-dot-Ag NP-PDMS films studied here (spectra on the right). Indeed,Figure 3 reveals that the Ag NPs assembled upon the filmsurface induced both dramatic quenching of C-dots’fluorescence as well as shifts in emission peak positions. Inaddition, changes in the intensity ratio among the spectra(excited at different wavelengths) were apparent (Figure 3).The pronounced quenching of the C-dots’ fluorescenceinduced by the Ag NPs likely corresponds to energy transferbetween the C-dots’ excitons and energy levels of the silver

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NPs.20,21 Such energy transfer processes, observed in other C-dot/Ag systems,31 attest to physical proximity between the AgNPs and the C-dots embedded within the PDMS matrix.9 Theshifts in emission maxima (shifts of the normalized spectra arehighlighted in Figure S3), and modulation of fluorescenceintensity ratio between the peaks (Figure 3, right spectrum) isconsistent with the above interpretation as the fluorescenceproperties of the C-dots are known to exhibit high sensitivity totheir close chemical environments.32

Deposition of Ag NPs upon the PDMS surface and energycoupling with the C-dots apparent in Figure 3 open the way toapplication of Surface-Enhanced Raman Scattering (SERS)applications. Figure 4 presents SERS experiments carried outusing the C-dot-Ag-NP-PDMS films as the sensing substrate.Figure 4A depicts SERS analysis of 4-amino-thiophenol (4-ATP), a conventional SERS-active dye,33,34 placed upon thePDMS film surface. Figure 4A clearly shows that the C-dot-Ag-NP-PDMS film enabled SERS detection as the typical spectrumof 4-ATP, dominated with the a1 vibrational modes (in-plane,in-phase modes), such as ν(CC) and ν(CS) at 1580 and 1080cm−1 respectively, as well as the clear observation of b2 modes(in-plane, out-of-phase modes) located at 1440 and 1143cm−1.33,34 Moreover, high sensitivity was achieved as 4-ATPgenerated strong SERS signals even in nanomolar concen-trations (Figure 4A). A SERS analytical enhancement factor(AEF) of 106 was calculated for the C-dot-Ag-NP-PDMS film.Notably, Figure 4A demonstrates that the combination of AgNPs and C-dots played a crucial role in achieving SERSdetection; no Raman peaks were recorded upon placing 4-ATPupon PDMS films comprising only C-dots, and no SERS wasobserved for PDMS films onto which Ag NPs wereindependently attached (i.e., without using the ascorbic acidprecursor, as outlined in Figure 1).Figure 4B demonstrates that the C-dot-Ag-NP-PDMS films

additionally constitute a useful SERS substrate for bacterial cells.The SERS pattern in Figure 4B was recorded followingdeposition of Pseudomonas aeruginosa bacterial cells in water (ata concentration of 104 CFU/mL) upon the C-dot-Ag-NP-PDMS film. Other bacterial species (Bacillus aureus, Erwiniaamylovora 238) were also measured and gave almost identicalSERS spectra. Close similarities between the SERS of differentbacteria have been often observed, and usually statisticaltechniques are needed in order to discriminate among bacterialspecies.35−38

The strong vibrations at 1400 and 930 cm−1 in the SERSspectrum in Figure 4B are assigned to carboxylate stretching,while the low frequency peaks likely correspond to SS andCS bonds in S-containing amino acids within bacterialproteins.35,36,39−42 While bacterial SERS data reported in theliterature vary, most are related to biomolecules at the bacterialcell surfaces in the proximity of the active sites of the metalsurfaces.35−38,43,44 Intriguingly, the SERS spectrum in Figure 4Bdoes not display the typical signature of bacterial cellmembranes, but rather echoes SERS analysis of bacterial cellsencapsulating silver colloids.43 This resemblance suggests thatcell surface destruction occurred upon the film surface,attributed to the abundant ascorbic acid moieties. Ascorbicacid is known to disrupt bacterial cell walls,35 consequentlyproducing vibrational modes of the inner components of thebacteria, apparent in the bacterial SERS spectrum (Figure4B).36,43

To evaluate the roles of the C-dots in generating the SERSresponse, and to specifically determine the significance of

Figure 2. Spectroscopic and microscopic characterization. A. X-rayphotoelectron spectroscopy (XPS) analysis. The assignment of peaksto specific atomic species is indicated in the spectra. The surveyspectra i and ii correspond to C-dot-PDMS film (without depositedsilver) and C-dot-Ag-PDMS film, respectively. B. Transmissionelectron microscopy (TEM, i) and high resolution TEM (HRTEM,ii) images of the nanoparticles extracted from PDMS. The latticespacing of the graphitic core of the C-dot (0.215 nm) metallic silver(0.235 nm) are indicated in ii. C. Scanning-TEM (STEM) image ofthe particles extracted from the PDMS film. The correspondingenergy-dispersive X-ray spectroscopy (EDS) acquired for therespective nanoparticles are indicated by the arrows. Scale bars in allimages correspond to 5 nm.

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coupling between the Ag NPs and C-dots, we recorded SERSspectra using films assembled through modified synthesisschemes (Figure 5). Specifically, in the SERS measurementsoutlined in Figure 5A we deposited 4-ATP upon PDMS filmscomprising the ascorbic acid precursor followed by Ag NPdeposition, but in which we altered the temperature of thehydrothermal step in the synthetic procedure (i.e., last step inthe scheme in Figure 1A). The preparation temperature iscritical, since the extent of carbonization (generating thegraphitic cores of C-dots) is dependent upon the synthesistemperature.45 Accordingly, modulation of the hydrothermaltemperature determined the relative abundance of C-dots in thecomposite film. In parallel, the Ag NP population was notsignificantly affected by the temperature changes.Indeed, Figure 5A demonstrates clear correlation between

the intensity of SERS signals and the hydrothermal temperatureemployed in film synthesis. This striking result supports theexistence of direct relationship between the SERS phenomenaand the abundance of C-dots embedded within the PDMS

films. For example, negligible SERS signals were apparent whenthe PDMS film was heated to 80 or 90 °C (Figure 5A)temperatures which are generally too low for production of C-dots from the ascorbic acid derivative precursor. However,pronounced SERS signals were recorded when 4-ATP wasplaced upon films prepared at 125 or 150 °C, sincetransformation of the carbonaceous precursors into C-dots ishighly efficient in such temperatures.9,45

The fluorescence spectra in Figure 5B confirm the directrelationship between synthesis temperature and relativeabundance of C-dots embedded within the PDMS matrix.Specifically, lower fluorescence signal (excitation 550 nm) wasapparent at a synthesis temperature of 80 °C (Figure 5B),reflecting a very small concentration of embedded C-dots. Incomparison, significantly more pronounced fluorescenceemissions were recorded at 125 and 150 °C, respectively,corresponding to greater abundance of C-dots present withinthe PDMS films. Overall, the fluorescence spectra in Figure 5B

Figure 3. Excitation-dependent emission spectra of the C-dots. Left: spectra recorded for C-dot-PDMS without Ag codeposited. Right: spectrarecorded for the C-dot-Ag NP-PDMS film. Note the significantly lower intensity of the fluorescence emission peaks.

Figure 4. Surface enhanced Raman scattering (SERS). A. SERS spectra of 4-ATP (1 nM) placed upon different film surfaces: C-dot-PDMS withoutAg NPs (spectrum i); Ag-NP-PDMS without C-dots (ii); C-dot-Ag-NP-PDMS film (iii). B. SERS spectrum of P. aeruginosa bacterial cells (104 CFU/mL) placed upon the C-dot-Ag-NP-PDMS film.

Figure 5. SERS analysis in different film compositions. A. SERS spectra of 4-ATP deposited upon C-dot-Ag-NP PDMS films constructed by usingdifferent temperatures (indicated) in the hydrothermal carbonization step (e.g., scheme in Figure 1A). B. Temperature-dependent fluorescenceemission spectra (exc. 550 nm) of the C-dot-Ag-NP-PDMS films producing the SERS signals in A. C. SERS spectra of 4-ATP placed upon PDMSfilms produced by spin coating. i. Ag NPs deposited via spin coating upon the PDMS film; ii. Ag NPs deposited upon C-dot-PDMS film via spincoating.

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corroborate the direct correlation between the SERS signalsand C-dots presence in the Ag-NP-coated polymer films.To further confirm the pivotal role of coupling between the

C-dots and Ag NPs for enabling the SERS phenomenonreported here, we examined SERS generation of PDMS filmsubstrates using Ag NPs deposition via spin coating (Figure5C). In these experiments, we recorded SERS signals uponplacing 4-ATP upon two types of films: PDMS film upon whichAg NPs were deposited via spin coating (Figure 5C,i), and afilm comprising PDMS mixed with preprepared C-dots, uponwhich Ag NPs were similarly deposited via spin coating (Figure5C,ii). Remarkably, C-dot-dependent SERS was recorded evenin the latter film system (Figure 5C,ii), while Ag NP-depositedupon PDMS films which did not contain C-dots were SERS-inactive (Figure 5C,i). SERS signals were similarly observedupon spin-coating Ag NPs upon polymer matrixes other thanPDMS (Figure S4), underscoring the generic nature of coupledC-dot-Ag NP film systems as SERS-active substrates.The SERS data in Figures 4 and 5 reveal direct correlation

between the SERS signals and the presence (and concen-tration) of C-dots in proximity to the Ag NPs deposited uponthe PDMS film. These results point to energy transfer from theC-dots to the Ag NPs as the underlying mechanism responsiblefor the Raman signals. Energy transfer processes between C-dots (as the donor) and Ag NPs (as the acceptor) werereported.31 Furthermore, the observation of SERS signals bothin case of SERS-active dyes as well as biological samples(bacteria) underscore the generic nature of the C-dot-Ag NPcoupling in the composite films.

■ CONCLUSIONSWe present construction of new composite polymer filmscomprising carbon dots and silver nanoparticles and their use asversatile SERS substances. The synthesis procedure utilized anascorbic acid amphiphilic derivative which plays a dual role inthe reactionboth constituting the carbonaceous buildingblock for generating C-dots as well as serving as the anchoringunit and reducing agent for Ag+ ions. Importantly, no otherreagents or reducing agents were added to the reaction mixture,thereby simplifying the synthetic route. The C-dot-Ag-NP-PDMS films were used as sensitive SERS sensing platformsboth in case of conventional SERS-active dyes as well as acomplex biological sample (bacterial cells). Crucially, theexperiments presented established a clear correlation betweenthe SERS signals and the presence of C-dots within the PDMSmatrix; this correlation likely arose from energy transferprocesses between the C-dots to proximate Ag NPs. Indeed,energy transfer between dif ferent NP species is considered afundamental aspect and a significant challenge in nanoparticleresearch and applications. Notably, the properties of the SERS-active composite material we constructed, which has not beendemonstrated before, correspond to the unique design of theflexible PDMS films embedding C-dots and coated with AgNPs.The analyte versatility and sensitivity of the C-dot-Ag-NP-

PDMS films are notable, since in many instances newlydeveloped SERS-based sensors could detect only conventionalSERS dyes, having limited applicability for sensing broad-basedmolecular targets (such as bacteria). Moreover, the detectionthreshold of the new composite film was low, reaching ananomolar detection threshold for 4-ATP and 104 cells/mL incase of the P. aeruginosa. The flexible film configuration isamenable for practical sensing applications through simple

placement of the target solution upon the film surface evenwhen biological samples are used.From a physical standpoint, the coupling between the C-dots

and Ag NPs is a unique core feature enabling SERS sensing.The experimental data demonstrate that the presence of the C-dots within the polymer film and their proximity to the Ag NPsare crucial determinants of the SERS signals observed. Indeed,maintaining proximate C-dots and Ag NPs in polymer filmsformed via different routes was a prerequisite for observingSERS. In conclusion, the C-dot-Ag-NP-PDMS films constitutea new SERS platform in which signals were directly related tocoupling between C-dots and Ag NPs. The films might be usedas versatile SERS sensor for detection of varied moleculartargets.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b10945.

Characterizations of materials (Figures S1−S4) (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: razj@bgu.ac.il (R.J.).Present Addresses§Department of Chemistry, Mahishadal Raj College, EastMidnapur, West Bengal 721628, India.∥Ruhr Universitat Bochum, Universitaetsstrasse 150, D-44780Bochum, Germany.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSS.K.B. is grateful to the Planning and Budgeting Committee(PBC) of the Israeli Council for Higher Education for anOutstanding Postdoctoral Fellowship. We thank Dr. VladimirEzersky for TEM analysis.

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10945ACS Appl. Mater. Interfaces 2016, 8, 25637−25643

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