Three-Dimensional Au-Coated Electrosprayed NanostructuredBODIPY Films on Aluminum Foil as Surface-Enhanced RamanScattering Platforms and Their Catalytic ApplicationsMehmet Yilmaz,†,‡,# Mustafa Erkartal,§,⊥,# Mehmet Ozdemir,§ Unal Sen,∥ Hakan Usta,*,§

and Gokhan Demirel*,†

†Bio-Inspired Materials Research Laboratory (BIMREL), Department of Chemistry, Gazi University, 06500 Ankara, Turkey‡Department of Bioengineering, Faculty of Engineering and Architecture, Sinop University, 57000 Sinop, Turkey§Department of Materials Science and Nanotechnology Engineering and ∥Department of Mechanical Engineering, Abdullah GulUniversity, 38080 Kayseri, Turkey⊥Siren Ultrasonik Research and Development, Erciyes Teknopark, 38039 Kayseri, Turkey

*S Supporting Information

ABSTRACT: The design and development of three-dimen-sional (3D) nanostructures with high surface-enhanced Ramanscattering (SERS) performances have attracted considerableattention in the fields of chemistry, biology, and materialsscience. Nevertheless, electrospraying of organic smallmolecules on low-cost flexible substrates has never beenstudied to realize large-scale SERS-active platforms. Here, wereport the facile, efficient, and low-cost fabrication of stableand reproducible Au-coated electrosprayed organic semi-conductor films (Au@BDY-4T-BDY) on flexible regular aluminum foil at a large scale (5 cm × 5 cm) for practical SERSand catalytic applications. To this end, a well-designed acceptor−donor−acceptor-type solution-processable molecularsemiconductor, BDY-4T-BDY, developed by our group, is used because of its advantageous structural and electrical properties.The morphology of the electrosprayed organic film changes by solution concentration, and two different 3D morphologies without-of-plane features are obtained. Highly uniform dendritic nanoribbons with sharp needle-like tips and vertically orientednanoplates (∼50 nm thickness) are achieved when electrospraying solution concentrations of 240 and 253% w/v (mg/mL) are,respectively, used. When these electrosprayed organic films are coated with a nanoscopic thin (30 nm) Au layer, the resultingAu@BDY-4T-BDY platforms demonstrate remarkable SERS enhancement factors up to 1.7 × 106 with excellent Raman signalreproducibility (relative standard deviation ≤ 0.13) for methylene blue over the entire film. Finally, Au@BDY-4T-BDY filmsshowed good catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol with rate constants of 1.3 × 10−2 and 9.2 ×10−3 min−1. Our results suggest that electrospraying of rationally designed organic semiconductor molecules on flexible substratesholds great promise to enable low-cost, solution-processed, SERS-active platforms.

KEYWORDS: Surface-enhanced Raman scattering, organic semiconductor, electrospraying, small molecule, catalysis

■ INTRODUCTION

Surface-enhanced Raman scattering (SERS) is one of the mostpowerful analytical tools in chemistry and materials science, andit shows high sensitivity in nondestructive label-free imaging ofvarious chemical species with a rapid response.1 In a typicalSERS technique, weak Raman scattering signals (scatteringcross section = 10−30 cm2/molecule) are intensified usingrationally designed metallic or inorganic semiconductor-basedsubstrates.1,2 The dominant mechanism behind the SERSprocess is the electromagnetic enhancement, which results fromthe amplification of electromagnetic fields around the analytemolecules by the excitation of localized surface plasmonresonances of a SERS-active substrate.2 The other commoncontributor to SERS enhancement is the chemical mechanism,which principally relies on charge transfer processes between

analyte molecules and the SERS-active surface.2−5 Since thediscovery of the SERS technique in 1970s,6 three-dimensional(3D) coinage metallic (e.g., Au, Ag, Cu) nanostructures haveattracted considerable attention as high-performing SERSsubstrates because the gaps and crevices known as “hotspots” in their morphologies enhance the local electromagneticfield and their large surface area allows improved adsorption ofanalyte molecules.2,6,7 The current techniques to fabricating 3Dhierarchical SERS substrates are based on either top-down orbottom-up nanofabrication methods.6 Although top-downmethods such as electron-beam lithography and nanoimprint

Received: March 2, 2017Accepted: May 8, 2017Published: May 8, 2017

Research Article

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© 2017 American Chemical Society 18199 DOI: 10.1021/acsami.7b03042ACS Appl. Mater. Interfaces 2017, 9, 18199−18206

lithography provide accurate control over the design andfabrication, these methods are costly and onerous. In the caseof the bottom-up strategy, micro-/nanostructured SERS-activeplatforms are typically produced from building blocks of noblemetallic nanoparticles.2,6,8,9 Although this strategy seems to besimple and inexpensive, the uncontrolled aggregation in mostcases leads to a discrepancy in the resulting SERS signals. Tothis end, it is still quite desirable to produce high-performance3D SERS-active platforms at a large scale via low-cost,reproducible, and facile methods.Electrospinning and electrospraying are simple, efficient, and

inexpensive techniques to fabricating micro-/nanostructuresfrom solution. When a certain electric field is applied to the tipof the syringe containing the electrospraying/electrospinningsolution, an electrically charged jet ejects from the Taylor coneformed in the vicinity of the tip and strikes toward thegrounded collector. By the effect of high voltage, the solutiongets thinned on the aluminum surface, increasing its surfacearea, which results in solvent evaporation and uniform nano-/microstructure formation on the collector.10−13 To this end,electrospinning of micro-/nanofibers has been restricted tohigh-molecular-weight polymeric solutions because of therequired entanglements and overlapping between polymerchains for continuous stretching of electrified jet and uniformfiber formation.10−16 However, recent studies indicate thatelectrospinning of nonpolymeric systems (i.e., small moleculeswith certain molecular weights or macromolecules) is alsopossible when sufficient intermolecular interactions existbetween molecular backbones.14,17,18 Until now, only a fewmolecular systems including phospholipids,14 surfactants, andcyclodextrins15 have been reported to show successful electro-spinning of fibers from solution. On the other hand,electrospraying of small molecules and macromolecules hasrecently attracted great attention as a promising method ofproducing micro-/nanostructured materials.19,20 Although,during a typical electrospinning process, fabricated micro-/nanofibers are deposited in the plane of the collector,electrospraying offers the great advantage of creating highlytextured 3D micro-/nanostructures that extend directly out ofplane, which offers the potential for SERS and catalysisapplications.21−23

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-basedsmall molecules with their excellent thermal/photochemical

stability, good solubility, intense absorption/emission profiles,high photoluminescence quantum yield, and small Stokes shiftsare used in a broad range of applications including fluorescentswitches, biochemical labeling,24 chemosensors,25 and electro-luminescent devices.26,27 In our previous study,28 we designedand synthesized a novel solution-processable molecular semi-conductor, BDY-4T-BDY, based on an acceptor−donor−acceptor (A−D−A) molecular π-architecture. The correspond-ing solution-processed films not only showed good semi-conductor performances in n-channel organic field-effecttransistors29−32 but also exhibited highly crystalline microfibersalong the shearing direction thanks to their strong intermo-lecular forces. This new BODIPY-based semiconductor isenvisioned as an attractive molecular building block tofabricating micro-/nanostructured soft materials so that Aulayers can be deposited on top via physical vapor deposition(PVD) to create a hybrid structure, which can be used for SERSand catalysis applications. The rationale to use BDY-4T-BDY isas follows: (i) the compound is highly soluble in commonorganic solvents for use in the electrospraying process, (ii) thecompound has directional molecular self-assembly and crystalgrowth tendency under mechanical stress, which might be alsoeffective under electrical stress to grow directional features viaelectrospraying; and (iii) the compound is an organicsemiconductor, and the electrospraying method may induceout-of-plane morphology growth as a result of its somewhatconductive nature at high voltage (12.5 kV). Furthermore,BDY-4T-BDY’s high chemical and thermal stabilities maycontribute to the robustness and stability of the correspondingSERS and catalysis platforms.Herein, in light of these findings, we present a facile approach

to fabricating flexible, stable, and reproducible gold-coatedelectrosprayed BDY-4T-BDY films on flexible regular alumi-num foil for practical SERS and catalysis applications (Scheme1). Electrospraying of BDY-4T-BDY solutions in a CHCl3/dimethylformamide (DMF) (2:1) mixture gave a highlyuniform dendritic nanoribbon-based morphology with sharpneedle-like tips and vertically oriented nanoplate-based (∼50nm thickness) morphology depending on the solutionconcentration. These micro-/nanostructured morphologieswere combined with a nanoscopic (30 nm) Au layer (Au@BDY-4T-BDY), which resulted in remarkable SERS perform-ances with enhancement factor (EF) up to 1.7 × 106 and

Scheme 1. Schematic for the Fabrication of Au-Coated Electrosprayed BDY-4T-BDY Organic Film on Aluminum (Au@BDY-4T-BDY)a

aThe first step shows the fabrication of micro-/nanostructured BDY-4T-BDY organic film via electrospraying, and the second step shows thedeposition of Au thin film (30 nm) by PVD.

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excellent reproducibility (relative standard deviation (RSD) ≤0.13). Additionally, catalytic activities of the current flexibleplatforms were tested for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), and the rate constants werefound to be (0.9−1.3) × 10−2.

■ EXPERIMENTAL SECTIONFabrication of Au-Coated Electrospun Micro-/Nanostruc-

tured BDY-4T-BDY Films. The synthesis, purification, and character-ization of BDY-4T-BDY were performed according to our reportedprocedure28 (Supporting Information). For the fabrication ofelectrosprayed micro-/nanostructures, BDY-4T-BDY solid (3.6 mgfor 240% w/v (mg/mL) solution and 3.8 mg for 253% w/v (mg/mL)solution) was weighed and dissolved in a solvent mixture of CHCl3 (1mL) and DMF (0.5 mL). Afterward, the solution was filtered througha poly(tetrafluoroethylene) filter (0.45 μm) and electrosprayed usingInovenso, NE300, consisting of a syringe with a stainless steel needle,an electrically ground collector, and a high voltage supply. Electro-spraying was conducted with a positive voltage of 12.5 kV applied tothe stainless steel needle with the solution feed rate of 80 μL/min witha syringe pump. The electrosprayed BDY-4T-BDY micro-/nanostruc-tures were collected on the collector covered with aluminum foil (5 cm× 5 cm) at a constant distance of 10 cm between the needle and thecollector. The as-prepared electrosprayed films were coated with ananoscopic layer of gold (30 nm) via the PVD method (NANOVAKHV, Turkey) under a fixed base pressure ((1 ± 0.2) × 10−6 Torr).Characterization of Au-Coated Electrospun Micro-/Nano-

structured BDY-4T-BDY Films. The ultraviolet−visible (UV−vis)absorption spectra of electrosprayed nano-/microstructured films wereobtained using a Shimadzu 2600 UV−vis−near-IR spectrophotometer.The absorption spectra of the films were obtained in the reflectivemode. 1H NMR spectra were recorded on a Bruker 400 spectrometer(400 MHz) and thin-layer chromatography (TLC) was performed onsilica gel plates coated with flourescent indicator F254. Themorphologies of the organic films were characterized by a ZeissEVO LS 10 field emission scanning electron microscope (SEM) aftercoating them with a thin layer of Au (∼5 nm). The IR spectra of theBDY-4T-BDY molecules and resultant films were collected via aFourier transform infrared (FTIR) spectrometer (Thermo Nicolet IR)operated in attenuated total reflection mode with a resolution of 4cm−1. Out-of-plane θ−2θ X-ray diffraction (XRD) measurements wereperformed using the Rigaku Ultima-IV X-ray diffractometer at a scanrate of 2°/min through diffracted intensity of Cu Kα radiation. Thecontact angle measurements were conducted on a drop shape analyzer(DSA100; Kruss, Germany), to record the shapes of the water dropletson the electrospun films.SERS and Catalytic Activity Studies. All SERS measurements

were conducted via the Raman microscopy system (Delta NuExaminer) equipped with a 785 nm laser source, a motorizedmicroscope stage sample holder, and a cooled charge-coupled devicedetector under the following experimental conditions: 20× objective, 3μm spot size, 30 s acquisition time, and 150 mW laser power.

Methylene blue (MB) was used as the Raman probe molecule, and, ina typical experiment, 5 μL of the aqueous solution of MB (1 mM) wasplaced on Au-coated films and retained until all of the solventevaporates. For each sample, at least 10 different Raman spectra werecollected from different spots across the substrate. For the catalyticstudies, Au-coated BDY-4T-BDY micro-/nanostructured films weretested for the catalytic conversion of 4-NP to 4-AP. In theseexperiments, Au-coated organic films were immersed into 9 mL of theaqueous solution of 1 × 10−4 M 4-NP and 0.033 M NaBH4. Thecatalytic conversion was monitored by UV−vis absorption spectros-copy through the decrease in the absorption band of 4-NP (λmax = 381nm) and emergence of the absorption band of 4-AP at λmax = 304 nm.

■ RESULTS AND DISCUSSIONS

Fabrication and Characterization of ElectrospunMicro-/Nanostructured BDY-4T-BDY Films. The synthesis,purification, and characterization of BODIPY-based smallmolecule BDY-4T-BDY were performed in accordance withour reported procedure (Scheme S1).28 Electrospraying of theBDY-4T-BDY solution in CHCl3/DFM (v/v 2:1) wasperformed onto regular aluminum foil (5 cm × 5 cm) inseveral different concentrations (240, 253, 270, 280, and 300%w/v (mg/mL)), which resulted in micro-/nanostructuredsurfaces with out-of-plane features. The SEM micrographs ofelectrosprayed BDY-4T-BDY surfaces are shown in Figures 1and S1. The morphology of the organic films was found tochange dramatically with concentration. As shown in Figure S1,higher concentrations (270−300% w/v (mg/mL)) lead tononuniform aggregate-based morphologies with very limitedout-of-plane features. This can be ascribed to the highconcentrations of the solutions, which probably facilitate theaggregation of organic small molecules during solventevaporation. When the solution concentration is controlled at253% (w/v) (mg/mL), mostly a vertically oriented nanoplate-based (∼50 nm thickness) morphology was obtained for BDY-4T-BDY(2). Finally, when the solution concentration wasfurther lowered to 240% (w/v) (mg/mL), a highly uniformdendritic nanoribbon-based morphology with sharp needle-liketips was obtained for BDY-4T-BDY(1). When these twomorphologies (Figure 1) are compared, the concentration isfound to play a critical role and a dilute solution (240% (w/v)(mg/mL)) is required to avoid aggregation-based particulatesand to obtain a uniform dendritic nanoribbon-based morphol-ogy. In contrary to typical electrosprayed small-molecule films,which usually have particulate-based morphological fea-tures,19,20 BDY-4T-BDY(1) films having dendritic nano-ribbon-based structures grown in the out-of-the-collectordirection build favorable 3D micro-/nanostructured morphol-

Figure 1. SEM images of electrosprayed organic films based on BDY-4T-BDY(1) (a) and BDY-4T-BDY(2) (b). The insets show highermagnifications, and the scale bars denote 2 μm and 200 nm.

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ogies for SERS and catalysis applications. During theelectrospraying process, although the flow rate is stable in thevicinity of the needle tip, as the solvent evaporates, it enters abending instability phase with further stretching of the solutionjet under electrostatic forces.22,23 When the conductivity of theelectrospraying solution increases, 3D structures extendingperpendicular to the collector surface can be formed.22 In ourwork, the formation of rather unusual 3D micro-/nano-structured electrosprayed morphologies might be attributedto somewhat conductive nature of the BDY-4T-BDY semi-conductor solution at high voltage (12.5 kV). Next, micro-/nanostructured BDY-4T-BDY films (5 cm × 5 cm) were coatedwith a nanoscopic Au layer (30 nm) via the PVD method,which shows that the 3D micro-/nanostructured organicmorphology is preserved after the PVD process and Auatoms are deposited on this micro-/nanostructured hydro-phobic surface as close-packed nanoparticles (∼15−20 nm insize) (Figure S2).θ−2θ XRD scans were performed to reveal out-of-plane

crystallinity of electrosprayed organic films, and the corre-sponding XRD spectra are shown in Figure 2. It is clearly

observed that both films are crystalline, showing almost thesame crystalline phases. The major diffraction peaks wereobserved at 2θ = 16.98−17.02°, indicating that electrosprayedBODIPY films employ a major crystalline phase with aninterlayer d-spacing of 5.2 Å along the substrate normal. Thepresence of high-angle diffractions as the major peaks inelectrosprayed films is consistent with those of solution-shearedfilms of BDY-4T-BDY.28 Note that this value is remarkablysmaller than the computed long-axis molecular length (∼2.4nm), and it is highly consistent with the computed length alongthe short molecular axis (∼7 Å).28 Thus, molecular packing ofBDY-4T-BDY molecules in the film phase most likely featureseither a highly tilted molecular orientation (θ > 75° from thesubstrate normal) on the substrate or molecular short-axisalignment along the substrate normal. Either way, this type ofmolecular orientation may induce strong “π···π” and “C−H···π”interactions in the out-of-plane direction, producing well-defined vertically oriented micro-/nanostructures. Indeed, theπ−π stacking interactions were evident in the XRD spectra ofBDY-4T-BDY(1), showing a broadened peak centered at 2θ ∼

24° (π−π stacking ∼ 3.7 Å), which is typical for films ofcrystalline small molecular semiconductors.33 In addition, asecondary crystalline phase was evident for BDY-4T-BDY(2)films with the primary diffraction peak at 2θ = 4.70° (d-spacing= 1.8−1.9 nm) along with higher-order diffractions at higherangles (14.18, 25.66, 44.98, and 65.18°). This crystalline phasemight refer to a typical edge-on molecular long-axis orientationon the substrate based on the computed molecular lengths(∼2.4 nm), and it might be associated with the formation of avertically oriented nanoplate-based (∼50 nm thickness)morphology on the surface.28,34

The UV−vis absorption spectra of electrosprayed filmsbefore and after gold deposition were recorded in the reflectivemode, and the corresponding spectra are shown in Figure 3.

Prior to the gold deposition, both films exhibit absorptionmaxima at 539−541 nm, which is mainly attributed to π−π*(S0 → S1) transition of the BODIPY moiety.35 In addition,higher energy peaks with lower intensities are observed at 380−385 nm, which corresponds to a combination of BODIPY-based S0 → S2 and oligothiophene-based π−π* transi-tions.28−30,36 When compared with the UV−vis absorptionspectra of BDY-4T-BDY recorded in tetrahydrofuran solution,the observed redshift for electrosprayed films in absorptionmaxima (Δλmax = 13 nm) and, more significantly, in the low-energy absorption onset wavelength (Δλonset = 100 nm)indicates π-core planarization and enhanced intermolecularinteractions in the solid state. This is consistent with theordered nature of these crystalline films (vide supra). After golddeposition, the vibronic peaks at 478−480 nm increase,whereas the absorption maxima at 539−541 nm are relativelylowered, and new absorption peaks with maxima located at710−712 nm appear, which is associated with the presence oflocalized surface plasmons of the top gold layer as a result ofthe micro-/nanostructured surface.After the electrospraying process, the electrosprayed film was

redissolved in CDCl3 and the chemical nature of the depositedmaterial was characterized by 1H NMR and TLC usingchloroform as the eluent, which confirms that the deposited

Figure 2. θ−2θ out-of-plane XRD pattern of electrosprayed organicfilms of BDY-4T-BDY(1) and BDY-4T-BDY(2).

Figure 3. UV−vis spectra of pristine electrosprayed organic films(BDY-4T-BDY(1) and BDY-4T-BDY(2)) and Au-coated electro-sprayed organic films (Au@BDY-4T-BDY(1) and Au@BDY-4T-BDY(2)).

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film consists of pure BDY-4T-BDY molecules (Figure S3). Inaddition, FTIR spectra of BDY-4T-BDY molecules and theirresultant electrosprayed films also reveal no chemical changeafter electrospraying (Figure S4). We also performed thecontact angle measurements to determine the wettability ofelectrosprayed films. As shown in Figure S4, the water contactangle of the electrosprayed organic films was found to be∼129−132°, which indicates highly hydrophobic surfacecharacteristics. When this film is compared with the drop-

casted organic film on glass (contact angle ∼88°), very highcontact angles of the electrosprayed films on aluminum shouldoriginate from the presence of fluorine substituents on anorganic π-framework and micro-/nanostructured morphology.It is noteworthy that after nanoscopic gold (30 nm) depositionthe films remain hydrophobic (∼127−131°), which isconsistent with the earlier studies on micro-/nanostructuredmetal surfaces.37,38

Figure 4. Representative SERS spectra of MB on Au@BDY-4T-BDY(1) and Au@BDY-4T-BDY(2). The Raman peaks at 1620 cm−1 used for EFcalculations are indicated on the spectra.

Figure 5. (a) Reproducibility of SERS spectra of MB collected on 30 randomly selected spots on Au@BDY-4T-BDY(1) film and the intensityhistogram of some prominent peaks at (b) 449, (c) 1437, and (d) 1620 cm−1. The dashed red lines indicate the average intensity of each peak.

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SERS and Catalytic Activity of Electrosprayed Nano-/Microstructured Films. The 3D micro-/nanostructuredsurface morphology of the current Au-coated electrosprayedorganic films over large areas (5 cm × 5 cm), when comparedwith smooth two-dimensional morphologies typically used inoptoelectronics, provide unique advantages, such as increasedsurface area, formation of hot spots, and capture of targetmolecules.8,39 Therefore, we envision that the current micro-/nanostructured organic films combining with a nanoscopic layerof gold (30 nm) would be a promising candidate for practicalSERS and catalytic applications. For SERS experiments, 5 μL ofthe aqueous solution of MB (1 × 10−3 M), which is used as thereporter molecule, was deposited on pristine BDY-4T-BDY filmand Au@BDY-4T-BDY film. Coin-shaped Raman samples withsizes of 2 ± 0.5 mm were obtained on the film surface. First, wetested the SERS performance of pristine organic films (FigureS6), which, as expected, did not yield any Raman signal. Thisresult is attributed to the absence of plasmonic metal to createelectromagnetic enhancement and unfavorable energy levels tolead to the chemical enhancement mechanism. However, aftergold deposition, well-defined and precise SERS signals werecollected with acceptable signal-to-noise ratios for bothmorphologies (Au@BDY-4T-BDY(1) and Au@BDY-4T-BDY(2)) (Figure 4). The most prominent peaks in the spectraare consistent with our earlier reports and can be summarizedas follows: 1620 cm−1 (ν(C−C) ring stretches), 1395/1437cm−1 (ν(C−N) symmetric and asymmetric stretches), and 449cm−1 (δ(C−N−C) skeletal deformation mode).8,40,41 Addi-tionally, to quantify and compare SERS performances of Au-coated electrosprayed organic films, we calculated the EF usingthe most prominent peak at 1620 cm−1 via the given formula

= ×

×− −

− −

N I

N I

EF ( )

/( )

n

n

reference Au@BDY 4T BDY( )

Au@BDY 4T BDY( ) reference

where Ireference and IAu@BDY‑4T‑BDY(n) are the Raman intensities ofMB on silicon wafer and the adsorbed MB on Au@BDY-4T-BDY(n), respectively, and Nreference and NAu@BDY‑4T‑BDY(n) arethe number of MB molecules for the reference sample andAu@BDY-4T-BDY(n), respectively. Accordingly, the EFs arecalculated to be 1.7 × 106 for Au@BDY-4T-BDY(1) and 3.6 ×104 for and Au@BDY-4T-BDY(2). For both cases, theseenhancements in SERS signals are attributed to the electro-magnetic enhancement mechanism. Electromagnetic contribu-tions to the SERS signals in these systems mainly stem fromhighly rough micro-/nanostructured surfaces of the currentplatforms, which employ closely spaced Au nanoparticles aswell (Figure S2). This unique 3D morphology decorated withnanostructured gold particles can generate hot spots with anextremely high electric field enhancement as a result of tip-focusing, cavity resonances, and antenna effects.8,40 As shown inFigure 4, the higher SERS peak intensity and EF obtained forAu@BDY-4T-BDY(1), as compared to those for Au@BDY-4T-BDY(2), might be attributed to the higher density of micro-/nanostructures and the presence of nanoribbon-like morphol-ogy with sharp needle-like tips, which is more favorable tocreate hot spots than nanoplates.The poor reproducibility of SERS signals has been one of the

main obstacles that should be addressed to realize an idealSERS substrate.40 To investigate the reproducibility of thecurrent Au-coated electrosprayed organic films, we collectedSERS spectra from 30 randomly selected spots on the surfaceprepared under identical conditions. The collected spectra for30 randomly selected spots and the corresponding intensitydistributions for the most prominent peaks at 449, 1437, and1620 cm−1 are summarized in Figure 5. To quantify and clarifythe reproducibility, the RSD values are calculated to be 0.13,0.10, and 0.11 for the peaks at 449, 1437, and 1620 cm−1,respectively. Our results clearly indicate excellent SERS

Figure 6. UV−vis absorption spectra of 4-NP as a function of time, indicating the catalytic activity of pristine and Au-coated electrosprayedsemiconductor films: (a) BDY-4T-BDY(1), (b) Au@BDY-4T-BDY(1), and (c) Au@BDY-4T-BDY(2), (d) percent conversion of 4-NP to 4-AP as afunction of time, (e) kinetics of the 4-NP reduction process assisted by gold-coated electrosprayed platforms.

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reproducibility of the current Au@BDY-4T-BDY platformsover large areas (Figure S7, 5 cm × 5 cm).In addition to SERS applications, we have also evaluated the

catalytic activity of Au-coated organic films for the reduction of4-NP to 4-AP. Although this catalytic conversion isthermodynamically possible in the presence of aqueousNaBH4, the kinetic barrier, due to the large potential differencebetween the borohydride donor (BH4

−) and acceptor (4-NP)molecules, limits the degree of this reaction.41,42 However, inthe presence of metal nanoparticles, electron relay from donorto acceptor molecules is easily actualized. The catalyticreduction of 4-NP can be effectively monitored via UV−visabsorption spectra (Figure 6). First, we employed the pristineBDY-4T-BDY organic film, and it resulted in a small decreasein the absorption peak at λmax = 381 nm with no emergence of4-AP (Figure 6a). This small change was attributed to favorablephysical adsorption of 4-NP molecules on the micro-/nanostructured organic film. However, when Au-coated organicfilms (Au@BDY-4T-BDY(1) and Au@BDY-4T-BDY(2)) wereemployed, we noted that the absorption at 381 nm graduallydecreased and a new peak appeared at 304 nm, indicating theformation of 4-AP. Also, an obvious color change from yellowto colorless could be visually observed in the solution due tothe catalytic conversion (Figure S8). For the case of Au@BDY-4T-BDY(1) films, almost complete conversion was achievedwithin 180 min. However, Au@BDY-4T-BDY(2) film led to arelatively lower catalytic activity with a significant retentiontime of ≈20 min. Considering pseudo-first-order reactionkinetics as shown in Figure 6e, the rate constants are found tobe 1.3 × 10−2 and 9.2 × 10−3 min−1 for Au@BDY-4T-BDY(1)and Au@BDY-4T-BDY(2), respectively. When compared tothat of Au@BDY-4T-BDY(2), the slightly higher catalyticactivity of Au@BDY-4T-BDY(1) films with negligible retentiontime can be attributed to its favorably higher density surfacemorphology with high-aspect-ratio dendritic nanoribbons. Notethat the rate constant for Au@BDY-4T-BDY(2) is calculatedafter the retention time of ≈20 min. These rate constants arecomparable to those of some recently reported Au-basedcatalysts in the literature (Table S1); however, large-scale andfacile fabrication of the current Au@BDY-4T-BDY substratesmay open new avenues for the development of recoverable,low-cost, and large-scale solid-state catalysis platforms.43−45

■ CONCLUSIONSIn summary, we have reported a simple and versatile approachto fabricating large-scale, flexible, and reproducible SERS-activesubstrates based on 3D micro-/nanostructured Au-coatedelectrosprayed semiconductor films (Au@BDY-4T-BDY). Ourfabrication method involves electrospraying and PVD, whichnot only provide a number of hot spots but also yield a largesurface area for the adsorption of analyte molecules. Owing tothe favorable organic morphologies achieved via simpleelectrospraying, remarkable SERS performances were achievedin terms of enhancement and reproducibility. Also, the Au-coated electrosprayed micro-/nanostructured films showedgood catalytic activity in the reduction of 4-NP to 4-AP.Taken together, our findings clearly show that electrosprayedmolecular semiconductor thin films, when coated with ananoscopic noble metallic layer, hold great promise for thefabrication of 3D SERS platforms. The bottom-up synthesis ofout-of-plane grown semiconducting micro-/nanostructures viathe electrospraying technique is ideal for the production of low-cost, flexible, and reliable large-scale SERS-active platforms and

catalysts. Moreover, by changing the structures of thesemiconductor molecules used, the SERS and catalyticperformances of the resulting platforms can be varied to realizefurther applications.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b03042.

Synthetic procedures and characterizations of BDY-4T-BDY: Scheme S1, Figures S1−S8, and Table S1 (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: hakan.usta@agu.edu.tr (H.U.).*E-mail: nanobiotechnology@gmail.com (G.D.).ORCIDMustafa Erkartal: 0000-0002-9772-128XHakan Usta: 0000-0002-0618-1979Gokhan Demirel: 0000-0002-9778-917XAuthor Contributions#M.Y. and M.E. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was partially supported by Gazi University (GrantNo: 05/2015-19) and the Scientific and TechnologicalResearch Council of Turkey (TUBITAK) grant number114M226. G.D. and H.U. acknowledge support from theTurkish Academy of Sciences, Distinguished Young ScientistAward (TUBA-GEBIP).

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