Silver-Nanoparticle Flexible SERS-Active Films

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Carbon-Dot/Silver-Nanoparticle Flexible SERS-Active Films Susanta Kumar Bhunia, Leila Zeiri, Joydeb Manna, Sukhendu Nandi, and Raz Jelinek ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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ACS Applied Materials & Interfaces

Carbon-Dot/Silver-Nanoparticle Flexible SERS-Active Films Susanta 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

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

ABSTRACT: Development of effective platforms for surface enhanced Raman scattering (SERS) sensing has mostly focused on fabrication of colloidal metal surfaces and tuning of their surface morphologies, designed to create “hot spots” in which plasmonic fields yield enhanced SERS

signals.

We

fabricated

distinctive

SERS-active

flexible

films

comprising

polydimethylsiloxane (PDMS) embedding carbon dots (C-dots) and coated with silver nanoparticles (NPs).

We show that the polymer-associated Ag NPs and C-dots intimately

affected the physical properties of each other. In particular, the C-dot/Ag-NP/polymer films exhibited SERS properties upon deposition of versatile targets, both conventional SERS-active dyes as well as bacterial samples. We show 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

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NPs, indicating that coupling between the plasmonic fields of the Ag NPs and C-dots’ excitons constituted a prominent factor in the SERS properties.

INTRODUCTION Metal surfaces, particularly gold and silver, constitute substrates for surface enhanced Raman scattering (SERS), a powerful sensing technology.1 The fundamental determinants of SERS activity are primarily traced to the morphology of the metal surface, specifically presence of nano-structured plasmon fields, especially around rough surfaces and sharp edges of metal colloids.2 To some extent, SERS phenomena were also affected by plasmon couplings and energy transfer processes involving metal or semiconducting substances.3 However, despite the considerable interest and vast potential applications of SERS-based sensors, there is still limited understanding and insufficient control of substrate features responsible for SERS signals. Most efforts towards forming SERS-active surfaces have focused upon modulating the nano-structural properties of metal surfaces aimed at generating SERS “hot spots”.1,2,4-6 Some studies have suggested that fabrication of hybrid surfaces comprising metal NPs and other nanostructures augmented charge-transfer processes contributing to more pronounced SERS signals.7,8 Carbon quantum dots (denoted “C-dots”) are promising new carbon nanoparticles exhibiting interesting photo-physical and electronic properties. C-dots encompass small fluorescent NPs (2 – 20 nm diameters) comprising graphitic cores surrounded by varied surface functional units.9,10 Surface properties of C-dots have been linked to their unique excitation-dependent emission properties.11,12

C-dots have attracted significant interest as a useful analytical and sensing

platform due to their broad color range, fluorescence brightness, stability, biocompatibility and low cytotoxicity, inexpensive and readily-available reagents, and simple synthesis procedures.13-


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Few studies have reported conjugation of C-dots with plasmonic metal NPs and the use of

such systems in photonic and sensing applications.19-23 Notably, C-dots in some of those coupled systems served also as reducing agents for generation of the metal NPs, exploiting electron donating groups at the C-dots’ surfaces.19-21 While in some instances the C-dots modulated the optical and spectroscopic properties of the noble metal NPs localized in close proximity, direct evidence for coupling between the plasmon properties of the metal NPs and C-dots’ excitons is still lacking. Furthermore, the C-dot/noble metal NP configurations reported thus far generally do not lend themselves to broad-based practical applications. Here we describe construction of SERS-active flexible polydimethylsiloxane (PDMS) films embedding C-dots and surface-attached silver NPs.

PDMS has been widely used as a

transparent, flexible polymer matrix for varied molecular and nanoparticle guest species.9,24,25. Recent studies have employed PDMS as a host matrix for photonic C-dot devices.9 The simple synthesis scheme developed here utilized an ascorbic acid derivative as both the carbonaceous building block for C-dot formation, as well as anchoring and reducing agent for generating surface-attached Ag NPs. The resultant flexible hybrid C-dot-Ag films displayed remarkable SERS properties, giving rise to intense signals both from conventional SERS probes as well as bacterial cells.

Notably, the experimental data demonstrate that the SERS signals were

specifically related to the presence of C-dots in proximity to the PDMS-bound Ag NPs, thus likely enabling energy and charge transfer processes between the two NP species. EXPERIMENTAL SECTION Materials. L-(+)-ascorbic acid, silver acetate, oleylamine, sodium sulfate, 4-amino thio phenol (4-ATP) and pyridine were purchased from Sigma Aldrich, USA. L-(+)-Tartaric acid was


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purchased from Alfa-Aesar, England. Lauroyl chloride was purchased from TCI, Japan. Chloroform,

toluene and

n-hexane were

bought

from

Daejung

chemicals,

Korea.

Tetrahydrofuran was purchased from Acros Organics, USA. Dimethyl formamide (DMF) and acetone were purchased from Frutarom (Haifa, Israel). Ethyl acetate and concentrated hydrochloric acid were purchased from Bio-Lab Ltd (Jerusalem, Israel). Precursors for PDMS film formation (Sylgard 184 silicone elastomer base and Sylgard 184 silicone elastomer curing agent) were purchased from Dow Corning Co., USA. All chemicals were used without further purification. Preparation of PDMS Films containing the C-dot Precursor. The carbon precursor 6-O-(OO′-dilauroyl-tartaryl)-L-ascorbic acid was synthesized according to a published report.26 Briefly, 142.3 mL of lauroyl chloride was added to 30 g of finely powdered L-tartaric acid in a round bottom flask under stirring and the reaction mixture was heated at 90 oC for 24 hours and then cooled to room temperature. The product was precipitated with n-hexane and dried under vacuum to obtain as white powder. To a solution of 20 g L-ascorbic acid in 75 mL anhydrous DMF 11.3 g of the above compound was added. The reaction mixture was cooled to 0°C and 1.83 mL of dry pyridine was added. Stirring was continued under argon atmosphere at 0°C for 30 minutes and then for 3 days at room temperature. After completion of the reaction the mixture was poured into 2N HCl at 0°C under vigorous stirring. The reaction mixture was extracted with ethyl acetate and the organic fraction was washed 3 times with brine, dried over Na2SO4, filtered and the solvent was removed under reduced pressure. The residue was precipitated twice with nhexane to obtain as white amorphous powder. 50 mgs of the ascorbic acid derivative were dissolved in 200 µL tetrahydrofuran (THF). 800 mg of Sylgard 184 silicone elastomer base (PDMS base) was mixed with 80 µL of the silicone elastomer curing agent in a falcon tube. The


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C-dot precursor compound was added into the falcon tube containing the PDMS film precursors mixed thoroughly and placed in a vacuum desiccator. The mixture was subsequently poured into a petri dish and maintained at 70-80 °C for 1 hour to make CDs precursor-PDMS film. Preparation of Ag-coated C-dot PDMS Films. 10 mg silver acetate was dissolved in 8 mL toluene with 75 µL oleyl amine. The PDMS film containing the C-dot precursor prepared according to the procedure above was dipped into the as-prepared silver salt solution for one minute and washed with toluene several times. The film was subsequently heated at 1250C for 2.5 hrs to form embedded C-dots. Solution Synthesis of C-dots. 50 mg of the ascorbic acid derivative and 165 µL de-ionized water were placed in glass vial and heated at 1250C for 2.5 hrs to obtain brown precipitate suggesting the formation of C-dots. The brown precipitate was then re-dispersed in 5 mL of chloroform through vortexing and centrifuged at 14,000 rpm for 30 min to remove high-weight precipitate and agglomerated particles. Chloroform was gently evaporated under reduced pressure to obtain a brown solid. The same procedure was repeated with 5 mL acetone, followed by solvent removal under reduced pressure to obtain monodisperse carbon dots. Spin-coating on Polymer Films. 800 mg of Sylgard 184 silicone elastomer base (PDMS base) was mixed with 80 µL of the silicone elastomer curing agent in a falcon tube. 30 mg of synthesized C-dots was dissolved in 200 µL THF and added into the falcon tube containing the PDMS film precursors mixed thoroughly and placed in a vacuum desiccator. The mixture was subsequently poured into a petri dish and maintained at 70-80 °C for 1 hour to make C-dotsPDMS film. 10 µL of hydroquinone reduced Ag NPs solution was put on C-dots-PDMS film and


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spin coated at 200 rpm for 20 minutes to get Ag NPs deposited on C-dots-PDMS film. Similar spin coating technique was carried out in case of polyvinyl alcohol (PVA) film. Characterization. C-dot-Ag-NP-PDMS films were immerged in ethyl acetate for extraction of carbon dots and Ag nanoparticles from the polymer films. High resolution transmission electron microscopy (HRTEM) samples were prepared by placing a drop of solution on a graphene-coated copper grid and observed with a 200 kV JEOL JEM-2100F microscope (Japan). HRTEM images were analyzed by fast Fourier transform (FFT). Scanning electron microscopy (SEM) experiments were conducted using a JEOL (Tokyo, Japan) model JSM-7400F scanning electron microscope. X-ray photoelectron spectroscopy (XPS) of C-dot-Ag-NP-PDMS film was performed using an X-ray photoelectron 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 with pass energy (PE) 150eV and high energy resolution spectra were recorded with pass energy (PE) 20eV. Processing of the XPS results was carried out using AVANTGE program. Fluorescence emission spectra of the C-dot-Ag-NPPDMS film at different excitation wavelengths were recorded on a FL920 spectrofluorimeter (Edinburgh Instruments, UK). Bacterial Growth. The bacteria used in the studies were Pseudomonas aeruginosa PAO1 wild type, Bacillus cereus, and Erwinia amylovora 238. Bacteria were grown aerobically at 37 °C in a sterilized solid 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 from each bacterial strain was taken and added to 10 mL sterilized LB medium and incubated at 37 °C. Bacterial growth was monitored at the desired time points through measuring the concentration of the bacteria by visible spectroscopy (108 CFU mL−1 when optical density at 600 nm was 1.0). Bacterial cells were


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separated from the growth medium through centrifugation followed by washing with water several times and taking suspension in water. To perform the SERS experiments, 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 and dried. SERS spectra were recorded with a Jobin-Yvon LabRam HR 800 micro-Raman system equipped with a Synapse CCD detector using He-Ne laser at 633 nm and Argon laser at 514.5 nm as excitation sources. The full laser power of 3mW on the sample was reduced by 10-100 using ND filters. The laser was focused with an x100 objective to a spot of about 1µ. The grating used was a 600 g mm-1 and the confocal hole of 100 µm with exposure times of 10-60 seconds. The analytical enhancement factor (AEF) was calculated according to the following equation:27

Where cRS corresponds for concentration of analyte which produces a Raman signal IRS under non-SERS condition. ISERS corresponds for Raman signal of the same analyte on SERS substrate using the experimental analyte concentration (cSERS). RESULTS AND DISCUSSION Figure 1 illustrates the synthesis scheme of the C-dot-Ag-NP PDMS films and their optical properties. We started with a mixture comprising the PDMS precursors (silicone elastomer base + curing agent) and an ascorbic acid derivative [6-O-(O-O′-dilauroyl-tartaryl)-L-ascorbic acid]. Following digestion at 60 0C a flexible PDMS film was formed, encapsulating the ascorbic acid


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amphiphiles. The ascorbic acid derivatives had a dual role in the synthesis, functioning both as the carbon precursors for generation of the C-dots, as well as reducing agents of Ag+ ions. Importantly, usage of the amphiphilic ascorbic acid derivative displaying covalently-attached hydrocarbon chains (Figure 1A) was essential in our experimental strategy, since the encapsulating PDMS matrix is hydrophobic. A recent study similarly demonstrated formation of flexible PDMS films embedding C-dots through co-dispersion of carbonaceous precursors and the elastomer base.9

Figure 1: Preparation of the C-dot-Ag-NP-PDMS films. A. Synthesis scheme. The ascorbic acid amphiphilic derivative (chemical structure shown on the left) was first mixed with the PDMS precursors, yielding a PDMS film following heating. Subsequent addition of Ag+ ions and heat-induced carbonization 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. following mixing with Ag+ ions – the yellowish color indicates Ag deposition; iii. after thermal treatment (carbonization); iv. Fluorescence image of the film (excitation 560 nm); v. photograph depicting film flexibility.

Following assembly of the PDMS films encapsulating the C-dot precursors they were placed in silver acetate organic solution, washed, and heated to 125 0C for 2.5 hours – yielding the final


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product - PDMS film embedding C-dots, with Ag particulates attached onto the film surface (Figure 1A). Figure 1B depicts photographs of the hybrid films, highlighting their fluorescence properties and physical flexibility. The initially-assembled PDMS film containing the ascorbic acid precursor is shown in Figure 1B,i. After silver deposition, the film had a yellowish brown appearance (Figure 1B,ii), reflecting the accumulation of metallic silver particles upon the film surface.19 Figure 1B,iii and iv demonstrates that the carbonized hybrid film displayed a darker brown color and red fluorescence (upon excitation at 560 nm) confirming formation of C-dots from the ascorbic acid derivative embedded within the film. The flexible and resilient nature of the produced films is shown in Figure 1B,v. Figures 2 and 3 present spectroscopic and microscopic characterization of the C-dot-Ag-PDMS films, particularly focusing on the nature of the carbon and silver particles formed through the synthesis scheme outlined in Figure 1A. The X-ray Photoelectron Spectroscopy (XPS) data in Figure 2A illuminate the atomic species present within the films. The XPS result of the C-dotAg-NP-PDMS in Figure 2A,ii shows the signature peaks of PDMS (Si 2p peaks), as well as confirms the formation of metallic silver upon the PDMS film surface (Ag 3d peaks at 368 eV and 375 eV, Figure 2A).28

The deconvoluted C 1s spectrum displays peaks at 284.7 eV

corresponding to sp2 carbon atoms (C=C) and 286.0 eV assigned to C−OH groups, whereas the deconvoluted O 1s spectrum reveals peaks at 532.0 eV for C=O and O=C-OH groups, and at 533.2 eV corresponding to C-OH and C-O-C groups. Importantly, the C 1s and O 1s XPS signals specifically correspond to atomic species present in C-dots,9 indicating that the carbonization process led to transformation of the PDMS-embedded ascorbic acid amphiphiles into C-dots. Indeed, the XPS of the C-dot-PDMS film (without Ag NP) in Figure 2A,i features C


9


1s and O 1s XPS signals at the same positions as the Ag-containing films, indicating formation of C-dots.9

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C-OH, C-O-C 531

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Si Cu

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KeV

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Figure 2: Spectroscopic and microscopic characterization. A. X ray photoelectron spectroscopy (XPS) analysis. The assignment of peaks to specific atomic species is indicated in the spectra. The survey spectra i and ii correspond to C-dot-PDMS film (without deposited silver) and C-dot-Ag-PDMS film, respectively. B. Transmission electron microscopy (TEM, i) and high resolution TEM (HRTEM, ii) images of the nanoparticles extracted from PDMS. The lattice spacing 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 of the particles extracted from the PDMS film. The corresponding energy-dispersive X-ray spectroscopy (EDS) acquired for the respective nanoparticles are indicated by the arrows. Scale bars in all images correspond to 5 nm.

The electron microscopy images in Figure 2B-C, recorded after dissolution of the PDMS film in ethyl acetate and extraction of the NPs, illuminate the sizes, crystallinity, and compositions of the carbon and silver nanoparticles produced through the synthesis process outlined in Figure 1A. The transmission electron microscopy (TEM) image in Figure 2B reveals relatively uniform NPs exhibiting diameters of between 2 nm – 5 nm. The high-resolution TEM (HR-TEM) image in Figure 2B reveals crystalline organizations of both the C-dots, displaying lattice spacing of 0.215 nm corresponding to the (110) planes of the graphite core, as well as the Ag nanoparticles


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exhibiting interplanar spacing of 0.235 nm corresponding to (111) lattice plane of Ag.10,29,30 The fast Fourier transform (FFT) diffraction patterns of the two nanoparticles confirm their crystalline lattices (Figure S1, Supporting Information). Scanning-TEM (STEM) analysis complemented by energy dispersive spectroscopy (EDS) in Figure 2C (full EDS maps are shown in Figure S2, Supporting Information) identified NPs comprising silver, corresponding to Ag NPs, while other particles did not compose of silver and thus were likely the C-dots. A central question pertaining to the new C-dot-Ag NP-PDMS system concerns the effects the Ag NPs and C-dots exert upon the physical properties of each other.

Figure 3 examines the

modulation of the C-dots’ photo-physical properties following silver deposition. Specifically, Figure 3 depicts the excitation-dependent fluorescence emission spectra of C-dot-PDMS films prepared without addition of Ag+ ions (i.e. no Ag NPs deposited, spectra on the left), and the Cdot-Ag NP-PDMS films studied here (spectra on the right). Indeed, Figure 3 reveals that the Ag NPs assembled upon the film surface induced both dramatic quenching of C-dots’ fluorescence as well as shifts in emission peak positions. In addition, changes in the intensity ratio among the spectra (excited at different wavelengths) were apparent (Figure 3). The pronounced quenching of the C-dots’ fluorescence induced by the Ag NPs likely corresponds to energy transfer between the C-dots’ excitons and energy levels of the silver NPs.20,21 Such energy transfer processes, observed in other C-dot/Ag systems,31 attest to physical proximity between the Ag NPs and the C-dots embedded within the PDMS matrix.9 The shifts in emission maxima (shifts of the normalized spectra are highlighted in Figure S3, Supporting Information), and modulation of fluorescence intensity ratio between the peaks (Figure 3, right spectrum) is consistent with the above interpretation as the fluorescence properties of the C-dots are known to exhibit high sensitivity to their close chemical environments.32


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3

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Wavelength (nm)

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

Deposition of Ag NPs upon the PDMS surface and energy coupling with the C-dots apparent in Figure 3 open the way to application of Surface-Enhanced Raman Scattering (SERS) applications. Figure 4 presents SERS experiments carried out using 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 the PDMS film surface. Figure 4A clearly shows that the C-dot/Ag-NP/PDMS film enabled SERS detection as the typical spectrum of 4ATP, dominated with the a1 vibrational modes (in-plane, in-phase modes), such as ν(CC) and ν(CS) at 1580 and 1080 cm-1 respectively, as well as the b2 modes (in-plane, out-of-phase modes) located at 1440 and 1143 cm-1 could be clearly observed.33,34 Moreover, high sensitivity was achieved as 4-ATP generated strong SERS signals even in nano-molar concentrations (Figure 4A). A SERS analytical enhancement factor (AEF) of 106 was calculated for the C-dotAg-NP-PDMS film. Notably, Figure 4A demonstrates that the combination of Ag NPs and Cdots played a crucial role in achieving SERS detection; no Raman peaks were recorded upon


13


placing 4-ATP upon PDMS films comprising only C-dots, and no SERS was observed for PDMS films onto which Ag NPs were independently attached (i.e. without using the ascorbic acid precursor, as outlined in Figure 1).

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4-ATP

iii

P. aeruginosa

Intensity (a.u.)

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Figure 4: Surface enhanced Raman scattering (SERS). A. SERS spectra of 4-ATP (1 nM) placed upon different film surfaces: C-dot-PDMS without Ag 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 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 following deposition of Pseudomonas aeruginosa bacterial cells in water (at a concentration of 104 CFU/mL) upon the C-dot-Ag-NP-PDMS film. Other bacterial species (Bacillus aureus, Erwinia amylovora 238) were also measured and gave almost identical SERS spectra. Close similarities between SERS of different bacteria have been often observed, and usually statistical techniques are needed in order to discriminate among bacterial species.35-38


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The strong vibrations at 1400 cm-1 and 930 cm-1 in the SERS spectrum in Figure 4B are assigned to carboxylate stretching, while the low frequency peaks likely correspond to S-S and C-S bonds in S-containing amino acids within bacterial proteins.35,36,39-42

While bacterial SERS data

reported in the literature vary, most are related to biomolecules at the bacteria cell surfces in the proximity of the active sites of the metal surfaces.35-38,43,44 Intriguingly, the SERS spectrum in Figure 4B does not display the typical signature of bacterial cell membranes, but rather echoes SERS analysis of bacterial cells encapsulating silver colloids.43 This resemblance suggests that cell surface destruction occurred upon the film surface, attributed to the abundant ascorbic acid moieties. Ascorbic acid is known to disrupt bacterial cell walls,35 consequently producing vibrational modes of the inner components of the bacteria, apparent in the bacterial SERS spectrum (Figure 4B).36,43 To evaluate the roles of the C-dots in generating the SERS response, and to specifically determine the significance of coupling between the Ag NPs and C-dots, we recorded SERS spectra using films assembled through modified synthesis schemes (Figure 5). Specifically, in the SERS measurements outlined in Figure 5A we deposited 4-ATP upon PDMS films comprising the ascorbic acid precursor followed by Ag NP deposition, but in which we altered the temperature of the hydrothermal step in the synthetic procedure (i.e. last step in the scheme in Figure 1A).

The preparation temperature is critical, since the extent of carbonization

(generating the graphitic cores of C-dots) is dependent upon the synthesis temperature.45 Accordingly, modulation of the hydrothermal temperature determined the relative abundance of C-dots in the composite film. In parallel, the Ag NP population was not significantly affected by the temperature changes.


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Raman Shift (cm-1)

Figure 5: SERS analysis in different film compositions. A. SERS spectra of 4-ATP deposited upon C-dot-Ag-NP PDMS films constructed by using different temperatures (indicated) in the hydrothermal carbonization step (e.g. scheme in Figure 1A). B. Temperature-dependent fluorescence emission 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 PDMS films produced by spin coating. i. Ag NPs deposited via spin coating upon the PDMS film; ii. Ag NPs deposited upon C-dot-PDMS film after spin coating.

Indeed, Figure 5A demonstrates clear correlation between the intensity of SERS signals and the hydrothermal temperature employed in film synthesis. This striking result supports the existence of direct relationship between the SERS phenomena and the abundance of C-dots embedded within the PDMS films. For example, negligible SERS signals were apparent when the PDMS film was heated to 800C or 900C (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 was placed upon films prepared at 1250C or 1500C, since transformation of the carbonaceous precursors into C-dots is highly efficient in such temperatures.9,45 The fluorescence spectra in Figure 5B confirm the direct relationship between synthesis temperature and relative abundance of C-dots embedded within the PDMS matrix. Specifically, lower fluorescence signal (excitation 550 nm) was apparent at a synthesis temperature of 80 0C


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(Figure 5B), reflecting a very small concentration of embedded C-dots.

In comparison,

significantly more pronounced fluorescence emissions were recorded at 125 0C and 150 0C, respectively, corresponding to greater abundance of C-dots present within the PDMS films. Overall, the fluorescence spectra in Figure 5B corroborate the direct correlation between the SERS signals and 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 phenomenon reported here, we examined SERS generation using PDMS film substrates prepared through spin coating, followed by deposition of Ag NPs (Figure 5B).

In these

experiments we recorded SERS signals upon placing 4-ATP upon two types of films: PDMS film upon which Ag NPs were deposited via spin coating (Figure 5C), and a film comprising PDMS mixed with pre-prepared C-dots, upon which Ag NPs were similarly deposited via spin coating (Figure 5C). Remarkably, C-dot-dependent SERS was recorded even in the latter film system (Figure 5C), while Ag NP-deposited upon PDMS films which did not contain C-dots were SERS-inactive (Figure 5C). SERS signals were similarly observed upon spin-coating Ag NPs upon polymer matrixes other than PDMS (Figure S4, Supporting Information), underscoring the generic nature of coupled C-dot-Ag NP film systems as SERS-active substrates. The SERS data in Figures 4-5 reveal direct correlation between the SERS signals and the presence (and concentration) of C-dots in proximity to the Ag NPs deposited upon the PDMS film. These results point to energy transfer from the C-dots to the Ag NPs as the underlying mechanism responsible for the Raman signals. Energy transfer processes between C-dots (as the donor) and Ag NPs (as the acceptor) were reported.31 Furthermore, the observation of SERS signals both in case of SERS-active dyes as well as biological samples (bacteria) underscore the generic nature of the C-dot / Ag NP coupling in the composite films.


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CONCLUSIONS We present construction of new composite polymer films comprising carbon dots and silver nanoparticles and their use as versatile SERS substances. The synthesis procedure utilized an ascorbic acid amphiphilic derivative which plays a dual role in the reaction – both constituting the carbonaceous building block for generating C-dots as well as serving as the anchoring unit and reducing agent for Ag+ ions. Importantly, no other reagents 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 platforms both in case of conventional SERS-active dyes as well as a complex biological sample (bacterial cells). Crucially, the experiments presented established a clear correlation between the SERS signals and the presence of C-dots within the PDMS matrix; this correlation likely arose from energy transfer processes between the C-dots to proximate Ag NPs.

Indeed, energy transfer between different NP species is considered a

fundamental aspect and a significant challenge in nanoparticle research and applications. Notably, the properties of the SERS-active composite material we constructed, which has not been demonstrated before, correspond to the unique design of the flexible PDMS films embedding C-dots and coated with Ag NPs. The analyte versatility and sensitivity of the C-dot-Ag-NP-PDMS films are notable, since in many instances newly-developed SERS-based sensors could detect only conventional SERS dyes, having limited applicability for sensing broad-based molecular targets (such as bacteria). Moreover, the detection threshold of the new composite film was low, reaching nanomolar detection threshold for 4-ATP and 104 cells/mL in case of the P. aeruginosa. The flexible film configuration is amenable for practical sensing applications through simple placement of the target solution upon the film surface even when biological samples are used.


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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 NPs, are crucial determinants of the SERS signals observed. Indeed, maintaining proximate C-dots and Ag NPs in polymer films formed via different routes was a pre-requisite for observing SERS. In conclusion, the C-dot-Ag-NPPDMS films constitute a new SERS platform in which signals were directly related to coupling between C-dots and Ag NPs. The films might be used as versatile SERS sensor for detection of varied molecular targets.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Characterizations of materials (Figure S1-S4) (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses ‡

Department of Chemistry, Mahishadal Raj College, East Midnapur, West Bengal 721628, India

╨

Ruhr Universitat Bochum, Universitaetsstrasse 150, D-44780 Bochum, Germany


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Dr. Susanta Kumar Bhunia is grateful to the Planning and Budgeting Committee (PBC) of the Israeli Council for Higher Education for an Outstanding Post-doctoral Fellowship. We thank Dr. Vladimir Ezersky for TEM analysis.

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Table of Contents Graphic

Ag+

300

900

1500

Raman Shift (cm-1)


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Silver-Nanoparticle Flexible SERS-Active Films

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