In-Situ Immobilization of Silver Nanoparticles on Self-Assembled

May 8, 2014 - detect trace-level analytes including pesticides, heavy metals, ... recently, honeycomb-patterned films via the breath figure (BF) metho...
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In-Situ Immobilization of Silver Nanoparticles on Self-Assembled Honeycomb-Patterned Films Enables Surface-Enhanced Raman Scattering (SERS) Substrates Yang Ou, Li-Yang Wang, Liang-Wei Zhu, Ling-Shu Wan,* and Zhi-Kang Xu MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Surface-enhanced Raman scattering (SERS) is an important sustainable technique because of its ability to detect trace-level analytes including pesticides, heavy metals, explosives, proteins, pathogens, and other chemical and biological contaminants. In this paper, we report a facile approach to highly sensitive SERS substrates by combining hierarchically patterned micro- and nanostructures with silver nanoparticles (Ag NPs) adsorbed on poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) surface. The patterned structures were fabricated using breath figures as dynamic templates in which polar block enriches at the rim of the pores. This characteristic of the breath figure method makes it possible to employ a simple coating technique to construct substrates decorated with Ag NPs, which have a diameter mostly ranging from 18 to 30 nm. Results indicate that the hierarchically ordered substrates show enhancement factors as high as 4 × 108. It is also found that the enhancement factors are concentration-dependent, relying on the ratio of the number of analytes to Ag NPs. Moreover, the prominent enhancement at Raman shift of ∼614 cm−1 has been observed, which will be useful in the detection of analytes such as some kinds of pesticides and viruses with characteristic peaks in this Raman shift range (e.g., adenovirus). This proposed approach provides an effective, reproducible, electroless, and facile method for preparing high-quality SERS substrates.

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INTRODUCTION Surface-enhanced Raman scattering (SERS) is an ultrasensitive vibrational spectroscopic technique to detect molecules on or near the surface of plasmonic nanostructures, yielding both qualitative and quantitative information based on the analytes’ SERS spectra. This technique has been applied to various fields including environment, health care, biology, and even terrorist threats because of its ability to detect trace-level analytes such as pesticides, heavy metals, explosives, proteins, pathogens, and other chemical and biological contaminants.1−13 It has been found as early as the 1970s that a few free-electron-like metals show SERS effect under the condition that the metal surface roughness or the colloid size is at the scale of several tens of nanometers. In the past two decades, boosted by the rapid development of nanotechnology, there has been tremendous progress in fabricating nanostructured surfaces as SERS substrates, such as colloidal metal nanoparticles (NPs), metal islands, fractal films, and ordered arrays.14−18 Although the fabrication of SERS substrates is no longer viewed as an insurmountable barrier, the key impediment for the practical use of SERS-based sensors is still the lack of robust and facile fabrication strategies for reproducible SERS substrates with large and stable Raman enhancement.3 It has been demonstrated that substrates with ordered structures show several advantages over disordered nanostructures; periodic arrays of NPs possess maximized specific surface © XXXX American Chemical Society

density of hot spots and higher surface enhancement factors (SEF). Genov et al. made theoretical and semiempirical studies of two-dimensional metal NP arrays and found that the ordered arrays can theoretically approach an average enhancement of 2 × 1011 for Ag nanodisk arrays.19 Various ordered SERS substrates have been fabricated, including close-packed metal NP arrays by self-assembly, metal nanostructures templated by colloidal crystals, and nanoporous arrays.20 For example, ordered Au films on nanostructured arrays templated by polystyrene microspheres enable detection of pesticide methyl parathion in a concentration as low as 10−10 M.20 Up to now, the most commonly used method to prepare ordered SERS substrates is still dependent on lithography techniques. More recently, honeycomb-patterned films via the breath figure (BF) method,21−42 which is a simple and fast method that uses condensed water droplets as dynamic templates, have also been used as SERS substrates.16−18 Hirai et al. combined the BF method and a vapor deposition process to produce Ag spike arrays, which show strongly SERS of rhodamine 6G (R6G).16 Tang and Hao prepared honeycomb films containing Au NPs by directional electrodeposition17 or from the hybrids.18 Received: March 31, 2014 Revised: May 3, 2014

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polymer in carbon disulfide (2 mg/mL, 50 μL) was cast onto the surface of PET film under a humid airflow at room temperature. The solution spreads well on PET film surface. The relative humidity of the airflow was maintained around 80% by bubbling through distilled water, which was measured using a hygrothermograph (DT-321S, CEM Corporation, Hong Kong). The flow rate was controlled at 2 L/min via a needle valve and was measured by a flow meter. After complete evaporation of the solvent and water (this process needs only about 1 min), a thin opaque film formed on the PET film surface was dried at room temperature. Honeycomb films with different pore diameters were prepared by controlling both the polymer concentration and the flow rate; higher concentration leads to smaller pore diameter for the studied system. Preparation of PS-b-PDMAEMA Flat Films. To compare with honeycomb films, dense flat films of PS-b-PDMAEMA were prepared by spin-coating the solution of the block polymer in toluene (30 mg/mL) onto a PET film. The spincoating was performed at a speed of 500 rpm for 6 s immediately followed by 3000 rpm for 60 s. The film was dried in vacuum at room temperature before use. Spin-Coating of PDMAEMA onto Honeycomb Films. PDMAEMA solution in ethanol (30 mg/mL, 0.5 mL) was spincoated onto a piece of as-prepared honeycomb film (∼4 cm2) at a speed of 500 rpm for 6 s immediately followed by 3000 rpm for 30 s. The films were dried at room temperature before use. In Situ Generation of Ag NPs on the Self-Assembled Honeycomb Films. First, AgNO3 aqueous solutions with different concentrations were made by dissolving AgNO3 in deionized water and by bubbling with N2 for at least 5 min before use. A honeycomb film was put face-down floating on the AgNO3 aqueous solution surface while the solution was kept bubbling N2. Subsequently, the film was quickly put facedown floating on the surface of NaBH4 aqueous solution that has the same molar concentration as the AgNO3 solution. To make the NaBH4 aqueous solution, NaOH was first dissolved in deionized water to adjust the pH to 10−12 into which a certain amount of NaBH4 was then added. Finally, the film was taken out from the NaBH4 solution, was thoroughly rinsed with deionized water, and was dried at room temperature. The same procedure was applied on the flat film. Measurements of Raman Spectra. Raman spectra were measured by a laser confocal Raman microspectroscopy (50 mW, inVia Reflex, Renishaw) with a 532 nm laser light as the excitation source with an integration time of 5 s and 0.5 mW input power (1%) for each spectrum unless otherwise stated. The diameter of the focused laser spot on the sample surface was about 1 μm. Typically, ∼20 μL ethanol solution of R6G was dropped onto films with an area of 1 cm2 and was dried under ambient conditions before measurements. Calculation of Surface Enhancement Factor (SEF). SEF was calculated according to the following equation:50

Interestingly, they found that more regular honeycomb films result in stronger SERS.18 In addition to the ordering of micro- or nanostructures, the distance dependence of the enhancement effect has also been reported, including the distance between adjacent nanostructures and that between analyte molecules and substrate. The SEF shows a near exponential decay with the gap distance between NPs.43 Recently, however, Bochterle et al. studied angstrom-scale distance dependence of antenna-enhanced vibrational signals and found no signal enhancement within the first 0.8 nm.44 This may imply that the NPs should be packed closely but not in zero distance. Similarly, results from polymer-coated substrates with Au or Ag colloids have revealed that SEF is strongly dependent on the coupling of nanoparticles. Glassy carbon grafted with poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) has been used to generate Ag NPs for SERS with an SEF of 6 × 106.45 Therefore, it is expected to fabricate ordered structures with NPs that are separated by polymer chains for highly sensitive SERS substrates. In our previous work, we found that film-forming materials containing polar moieties are able to form hierarchically structured honeycomb films through the self-segregation during the BF method.46−48 Herein, we present a facile approach to SERS substrate which combines ordered hierarchical structures and Ag NPs generated in situ. The substrates show exciting SEFs as high as 4 × 10 8 . This work also provides straightforward evidence for the chemical enhancement of SERS.

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EXPERIMENTAL SECTION Materials. The synthesis of polystyrene-block-poly(N,Ndimethylaminoethyl methacrylate) PS247-b-PDMAEMA14 (Mn = 27 900 g/mol, Mw/Mn = 1.24) by atom transfer radical polymerization (ATRP) was described elsewhere.49 N,NDimethylaminoethyl methacrylate (DMAEMA) was commercially obtained from Sigma-Aldrich and was distilled under reduced pressure before use. Azobis(isobutyronitrile) (AIBN) was recrystallized in ethanol at 40 °C. Silver nitrate (AgNO3, 99.99%), NaBH4 (98%), and Rhodamine 6G (R6G, 95%) were purchased from Aladdin and were used as received. Poly(ethylene terephthalate) (PET) film was kindly provided by Hangzhou Tape Factory and was cleaned with acetone for 2 h before use. Water used in all experiments was deionized and ultrafiltrated to 18.2 MΩ with an ELGA LabWater system. All other reagents were acquired from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and were used without further purification. Synthesis of PDMAEMA. PDMAEMA (Mn = 38 800 g/ mol, Mw/Mn = 2.18) was synthesized by free radical polymerization. Briefly, the polymerization of DMAEMA (10 mL) was initiated with AIBN (194 mg) in ethanol (10 mL) at 50 °C. Before the reaction proceeded, the solution was bubbled with N2 for 30 min at room temperature. After about 6 h, into the nonflowing solution 40 mL of ethanol was added to dissolve the resultant polymer. The homogeneous solution was then dropped into a large amount of petroleum ether (∼200 mL) with stirring. This dissolution−precipitation process was repeated three times. The obtained sticky polymer was dried in vacuum at ambient temperature. Preparation of Honeycomb Films. Honeycomb-patterned films were prepared via the breath figure method according to our reported procedure.49 Typically, a solution of

SEF = (ISERS/NSERS)/(Ibulk /Nbulk )

where ISERS and Ibulk denote the integrated intensities for the strongest band of the R6G molecules adsorbed on the film surface and those dissolved in solution, respectively, whereas NSERS and Nbulk represent the average number of R6G molecules in the scattering volume excited by the laser beam. Characterization. The molecular weight of PDMAEMA and its distribution was measured by a PL 220 gel permeation chromatography (GPC) instrument at 25 °C, which was B

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equipped with a Waters 510 HPLC pump, three Waters Ultrastyragel columns (500, 103, and 105 Å), and a Waters 410 DRI detector. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1.0 mL/min. The calibration of the molecular weights was based on polystyrene standards. The surface morphology of the samples was observed using a field emission scanning electron microscope (FE-SEM, 3 kV, S-4800, Hitachi) with energy-dispersive X-ray spectrometry (EDX, 20 kV). Mapping based on Ag element was conducted on the film surface within a selected area. Except samples with Ag NPs, the samples were precoated with platinum by an ion sputter prior to imaging.

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RESULTS AND DISCUSSION The SERS substrates are based on honeycomb-patterned porous films prepared by the BF method (Figure 1). Previously,

Figure 2. SEM images of (a, b) an as-prepared honeycomb film recorded at different magnifications and (c) the film immobilized with Ag NPs. (d) EDX spectrum and the mapping image of Ag element on the honeycomb film with Ag NPs.

Figure 1. Schematic of the formation of honeycomb films with polymer-coated Ag NPs for SERS substrates.

also exist in the pores. It was estimated from SEM images taken at high magnification that most of the NPs have a diameter ranging from 18 to 30 nm; a few of them are larger but are less than 100 nm (interestingly, they are mainly located at the rim of the pores). Although Ag NPs formed via some other methods such as electrochemical deposition may be more uniform,52 the Ag NPs are suitable for SERS as it is believed that SERS-active systems must possess features in the range of 5−100 nm while the strongest SERS effects are usually observed from NPs with a size of 20−70 nm.3 Results of energy-dispersive X-ray spectrometry (EDX) confirmed the formation of patterns and the uniform distribution of Ag NPs (Figure 2d). Compared with the well-established BF process, the key step in the present method is the adsorption of Ag+. As mentioned above, a layer of PDMAEMA is formed on the film surface by coating instead of by covalent immobilization. Although the coated PDMAEMA can be stabilized to a large degree by the film-forming material PS-b-PDMAEMA, it will be slowly dissolved into the AgNO3 aqueous solution. On the other hand, the stability of the PDMAEMA layer is improved in the reduction step after immobilizing Ag NPs; in this step, the Ag NPs act as a cross-linker. Therefore, controlling the concentration of AgNO3 solution and the adsorption time is crucial to high-quality SERS substrates. As shown in Figure 3,

we fabricated highly ordered honeycomb films via the BF method, which show promising potentials in size-selective separation,51 biocatalysis,48 biosensing,47 biomolecule microarrays,46 and controlled assemblies of NPs.49 Block copolymer polystyrene-b-PDMAEMA (PS-b-PDMAEMA)49,51 was dissolved in CS2 to make a homogeneous solution and was cast on supporting films under a moist airflow. Evaporative cooling leads to condensation of water droplets, which act as dynamic templates for large area highly ordered porous films (Figure 2a, b). A thin layer of PDMAEMA was then coated onto the resultant honeycomb films for generating Ag NPs in situ. Previously, various kinds of polymers were used for preparing honeycomb films; here, PS-b-PDMAEMA is used because it is speculated that the PDMAEMA block is able to interact with the PDMAEMA coating layer and, hence, increase the adsorption of PDMAEMA onto the honeycomb film surface. Electron-deficient Ag+ can be adsorbed along the PDMAEMA chains by electrostatic interaction.45 Subsequently, reduction of the coordinated Ag+ in aqueous NaBH4 solution generates Ag NPs adsorbed on PDMAEMA surface. Figure 2c shows SEM images of substrates fabricated at optimal conditions. It is clear that the external surface of the honeycomb films is covered with NPs. If we observe carefully or remove the top surface of the films, it can be found that NPs C

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Figure 3. Surface enhancement factors (SEFs) of honeycomb filmbased SERS substrates prepared with different AgNO3 concentrations and adsorption time ([NaBH4] = [AgNO3], reduction time = 5 min).

SEFs of the substrates increase with AgNO3 concentration first and then decrease, and the decrease is remarkable especially for a long adsorption time. At low AgNO3 concentration (0.1 and 0.2 M), SEFs increase greatly with adsorption time; longer adsorption time (>20 min) may lead to saturation of adsorption. However, it is reverse at high AgNO3 concentration (0.4 and 0.6 M). It is observed that Ag NPs aggregate at high AgNO 3 concentration (Figure S1a in the Supporting Information), resulting in weakened enhancement. It can be attributed to less hot spots produced by NP dimer and worse accessibility of analyte to the Ag NP aggregates.53 In addition, substrate prepared at 0.1 M AgNO3 for 5 min shows a very poor spectrum because of insufficient immobilized NPs (Figure S1b of the Supporting Information). Therefore, substrates used for the following detection were prepared at optimal conditions, that is, 20 min in 0.2 M AgNO3 solution followed by 5 min in 0.2 M NaBH4 solution. The honeycomb films were also fabricated under optimal BF conditions; as a result, the film shows well-patterned pores in an area of at least 2 cm2 with reproducible and adjustable pore size. We studied the reproducibility of the SERS substrates (Figure S2 of the Supporting Information). Results from parallel samples indicate that the substrates are well reproducible. However, Ag NPs have been reported to be instable under ambient conditions at relative humidities greater than 50%,54 which may restrict the long-term use of such substrates. Effects of pore diameter of the films on the enhancement of SERS were investigated. On the basis of our previous work,46−48,51 films with different pore diameters were obtained by changing the polymer concentration and flow rate. The SEFs of SERS substrates with pore diameters of 3.8, 3.0, and 2.2 μm prepared at the optimal conditions are 4.1 × 106, 4.6 × 106, and 5.6 × 106, respectively, which were measured at an R6G concentration of 10−6 (Figure 4). Films with smaller pores possess slightly higher enhancement because smaller pores lead to larger specific surface area, which will increase the number of immobilized Ag NPs. Nevertheless, it can be seen that the pore diameter of honeycomb films only shows very little influence on the enhancement. For comparison, flat PS-b-PDMAEMA film was prepared for immobilizing Ag NPs using the same procedure. These

Figure 4. (a, b) SEM images of Ag-immobilized honeycomb films with different pore diameters. (a) 3.0 μm and (b) 3.7 μm. (c) The corresponding Raman spectra on films with different pore diameters of 2.2, 3.0, and 3.8 μm. The concentration of R6G is 1.0 × 10−6 M ([AgNO3] = 0.2 M, adsorption time = 20 min). The SEM image of the film with a pore size of 2.2 μm is shown in Figure 2.

substrates were used to detect R6G, a typical model analyte. The SERS spectra are shown in Figure 5. All the spectra were

Figure 5. Raman spectra of R6G solutions with different concentrations on Ag NPs-immobilized flat and honeycomb films.

collected on a laser confocal Raman microspectroscopy with a 532 nm laser light as the excitation source. The laser spot was focused to about 1 μm, the input power was 0.5 mW (1%), and the integration time was 5 s for each spectrum. It can be seen from Figure 5 that the flat film with Ag NPs shows very weak enhancement but that the honeycomb film has a very high SEF when 10−6 M R6G solution was used. Meanwhile, the fluorescence background increases. The topography of the honeycomb surface leads to more uniform and a larger amount of adsorbed PDMAEMA and, hence, more uniform and a larger amount of Ag NPs (Figure 2c and Figure S1 of the Supporting Information). D

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depending on the ratio of the number of analytes to Ag NPs. We also observed possible chemical enhancement on the patterned substrates induced by the molecular orientation of analytes on the patterned substrates. This highly effective, reproducible, electroless, and facile method is useful in preparing high-quality SERS substrates.

Most importantly, our previous results demonstrated that in the BF process hydrophilic blocks can enrich at the pore wall, that is, water/solution interface, to form hierarchical nanostructures.46−48 The PDMAEMA block at the outmost surface of honeycomb films also favors the adsorption of PDMAEMA homopolymer. Instead, the flat film surface will be enriched with hydrophobic PS to decrease the energy at the air interface. This is why a large amount of Ag NPs form on honeycomb film especially at the rim of the pores. The quantity of Ag NPs on honeycomb films is still much smaller than that on substrates fabricated by electroplating as evidenced by EDX analysis (see intensity ratio of Ag to C from Figure 2d in this work and, for example, Figure 1e in ref 16). On the other hand, the laser beam size we used (∼1 μm) is smaller than the periodicity of the honeycomb structure with NPs. However, our ordered substrates show comparable or even higher SEFs. The hierarchical nanostructures as well as the honeycomb microstructures are responsible for the great enhancement. It is interesting that the SEFs show a dependence on the concentration of R6G. When 10−9 M R6G was used, the SEF obviously increases to 4 × 108, which is an exciting enhancement compared with those reported in the literature. Considering 20 μL R6G solution was dropped onto ∼1 cm2 film in this work, it can be calculated that there are ∼1.2 × 105 R6G molecules per μm2 if 10−6 M R6G is used. It can also be estimated from the SEM images that the density of Ag NPs is about 1.0 × 103 per μm2. In other words, the density of R6G is far larger than Ag NPs, that is, ∼120 R6G molecules per Ag NP. However, if 10−9 M R6G is used, one R6G molecule can theoretically occupy about eight Ag NPs. This means a much larger possibility for R6G molecules to be located at the hot spots and a much closer distance between R6G and Ag NPs, leading to greater enhancement. Similar concentration dependence has been reported by other groups.55,56 It is also interesting that our substrates reveal possible chemical enhancement. Two classes of SERS mechanisms, electromagnetic and chemical, have been accepted. Previous results demonstrated that one certain substrate may show different enhancements for different molecules.3,4 Here, we found different enhancements for different vibrational modes in one molecule. Compared with SERS spectra reported by Hirai et al.16 and Gupta et al.,45 where 1375 and 1680 cm−1 because of the aromatic C−C stretching vibrations were found to be the strongest peaks, respectively, our spectra show that the peak at 614 cm−1 arising from C−C−C ring in-plane bending vibration (Table S1 of the Supporting Information) shows much stronger enhancement (Figure 5).57 It may be induced by the molecular orientation of analytes on the patterned substrates.55,56 However, it is still unknown and needs further investigation. This special enhancement at Raman shift around 614 cm−1 will be helpful in the detection of analytes such as some kinds of pesticides and viruses with characteristic peaks in this range, for example, adenovirus.58

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ASSOCIATED CONTENT

S Supporting Information *

Effects of uniformity, reproducibility, and characteristic Raman peaks of R6G. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +86-571-87953763. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21374100 and 51173161) and the Fundamental Research Funds for the Central Universities (2014QNA4037).

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CONCLUSIONS In summary, we report the fabrication of hierarchically ordered SERS substrates by combining the BF method and in situ generated Ag NPs adsorbed on PDMAEMA surface. It has been demonstrated that the concentrations of precursors as well as the adsorption time are important to the amount and uniformity of Ag NPs, which in turn have a great impact on the enhancement factor. Substrates prepared under optimal conditions have enhancement factors as high as 106∼108, E

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