Article pubs.acs.org/Langmuir
Comparative Study of the Self-Assembly of Gold and Silver Nanoparticles onto Thiophene Oil Manuel Gadogbe,† Siyam M. Ansar,† I-Wei Chu,‡ Shengli Zou,§ and Dongmao Zhang*,† †
Department of Chemistry and ‡Institute for Imaging and Analytical Technologies, Mississippi State University, Mississippi State, Mississippi 39762, United States § Department of Chemistry, University of Central Florida, Orlando, Florida 32816, United States S Supporting Information *
ABSTRACT: Nanoparticle self-assembly is fundamentally important for bottom-up functional device fabrication. Currently, most nanoparticle selfassembly has been achieved with gold nanoparticles (AuNPs) functionalized with surfactants, polymeric materials, or cross-linkers. Reported herein is a facile synthesis of gold and silver nanoparticle (AgNP) films assembled onto thiophene oil by simply vortex mixing neat thiophene with colloidal AuNPs or AgNPs for ∼1 min. The AuNP film can be made using every type of colloidal AuNPs we have explored, including sodium borohydride-reduced AuNPs with a diameter of ∼5 nm, tannic acid-reduced AuNPs of ∼10 nm diameter, and citrate-reduced AuNPs with particle sizes of ∼13 and ∼30 nm diameter. The AuNP film has excellent stability and it is extremely flexible. It can be stretched, shrunken, and deformed accordingly by changing the volume or shape of the enclosed thiophene oil. However, the AgNP film is unstable, and it can be rapidly discolored and disintegrated into small flakes that float on the thiophene surface. The AuNP and AgNP films prepared in the glass vials can be readily transferred to glass slides and metal substrates for surface-enhanced Raman spectral acquisition.
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liquid film to prevent AuNP precipitation.27 Indeed, following this line of thinking, Kowalczyk et al. successfully prepared AuNP-armored toluene droplets by using dithiol as a crosslinker of the AuNPs on the oil surfaces.27 In comparison to the extensive literature on AuNP selfassembly, information on AgNP self-assembly has been scant. Dai et al. reported the assembly of dodecanethiol-capped AgNPs onto the oil/water interface with oil droplets below 10 μm in diameter.28 Yu et al. also reported the 2D self-assembly of a poly(vinylpyrrolidone)-stabilized Ag nanowire on the chloroform/water interface in a plastic vial.29,30 Recently, Konrad et al. reported the self-assembly of citrate-reduced AgNPs at dichloromethane/water interfaces in the presence of tetrabutylammonium nitrate which acts as the promoter of the self-assembly.31 Reported herein is our finding that both AuNPs and AgNPs in water can easily self-assemble onto thiophene/ water and thiophene/glass interfaces, forming an AuNP or AgNP film completely covering the thiophene surface in thiophene/water mixtures in glass vials without the need for any surfactant, polymer, or cross-linker as the NP surface modifier or an exogenous promoter. This work also represents the first head-to-head comparison of the assembly and properties of AuNP and AgNP films on water/oil interfaces.
INTRODUCTION The self-assembly of nanoparticles (NPs) into higher-order structures is a promising technique for the bottom-up fabrication of advanced functional materials.1,2 With their unique chemical and electromagnetic properties, colloidal gold nanoparticles and silver nanoparticles (AuNPs and AgNPs) exhibit enormous potential for a wide range of applications including solar energy harvesting, environmental processing, biosensing, and medicine.3,4 Various higher-order AuNP structures have been reported,5−8 which include linearly patterned AuNPs (1D)6,8 and a 2D NP film on a solid support9 and at the liquid/liquid or liquid/air interface.10−15 Long-range-ordered planar 2D NP arrays on solid substrates are commonly fabricated using an evaporative self-assembly process16−21 or surface-functionalized substrates,22 whereas planar NP films at liquid/liquid interfaces have been prepared in situ by synthesizing NPs at water/oil interfaces.13,23 Threedimensional complete surface coverage was first reported by Duan et al.,24,25 who showed that AuNPs and AgNPs capped with 2,2′-dithiobis [1-(2-bromo-2-methylpropionyloxy) ethane] can self-assemble into a thin film completely enclosing the water surface of water−toluene mixtures in plastic Eppendorf tubes. Recently, micrometer-sized silica−gold Janus particles with anisotropic wettability have been prepared for stabilizing oil droplets in water.26 Kowalczyk et al. speculated that due to the lack of surfactant properties, spherical AuNPs need to be covalently cross-linked on the © 2014 American Chemical Society
Received: June 30, 2014 Revised: August 13, 2014 Published: September 8, 2014 11520
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room temperature. The SERS spectra were acquired using a 633 nm He−Ne laser. The laser power used for spectra acquisition for the airdried and oven-heated films was 1.3 mW, and the spectral integration time was 500 s. A 5 μL sample of 5 and 10 μM rhodamine 6G (R6G) solution was then deposited onto the AgNP and AuNP films, respectively, after heat treatment, and the SERS spectra of R6G were acquired using a 785 nm laser. The laser powers used for the R6G SERS spectra are 0.4 and 1 mW for AgNP and AuNP films, respectively. The spectra acquisition time for all R6G SERS spectra is 500 s.
Such a study is important for understanding AgNP and AuNP similarities and differences in the phase partitioning. Recent research from our group showed that AgNPs and AuNPs differ significantly in their interfacial interactions with organothiols.32,33
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EXPERIMENTAL SECTION
Materials and Equipment. All chemicals were purchased from Sigma-Aldrich except for thiophene (lot no. 10164877, 99%) that was purchased from Alfa Aesar. The sodium borohydride-reduced AuNPs and the ∼13 and ∼30 nm citrate-reduced AuNPs were synthesized in house according to published procedures,34−36 but the 10 nm tannic acid-reduced AuNPs were purchased from Nanocomposix Inc. The citrate-reduced AgNPs were also synthesized in house using the Lee− Misel method.37 Nanopure water was used throughout the experiments. The RamChip slide used for the SERS spectral acquisition was obtained from Z&S Tech LLC. The RamChip is a normal Raman substrate that is both fluorescence- and Raman-background-free.38,39 The normal Raman and surface-enhanced Raman spectroscopy (SERS) spectra were obtained with a Horiba LabRam HR800 confocal Raman microscope system and a 633 nm Raman excitation laser. UV− visible measurements were taken using a Fisher Scientific Evolution 300 UV−visible spectrophotometer. Centrifugation was performed using a benchtop Fisher Scientific centrifuge (Fisher 21000R). Sonication was performed using a Branson Sonifier 250. The vortex mixer (SI-T236) was acquired from Scientific Industries Inc. The vortex mixer speed is in revolutions per minute (rpm). General Procedure for the Preparation of AgNP- and AuNPEnclosed Thiophene. Unless specified otherwise, all of the AgNPand AuNP-covered thiophene was prepared with the following procedure. A volume of AuNP or AgNP solution is mixed with a volume of neat thiophene (99%) in a glass vial, and the resulting mixture is vortex mixed for ∼1 min using a benchtop vortex machine set at a speed of ∼3200 rpm. The mixture is then allowed to sit at room temperature. Complete AuNP or AgNP film formation on the thiophene surface typically occurs within ∼2 min of sample preparation. Normal Raman and SERS Measurements for Thiophene and AuNP-Enclosed Thiophene. The normal Raman spectrum of neat thiophene was acquired by focusing the laser on a sample of neat thiophene in a 4 mL glass vial. For the thiophene SERS spectrum, a small volume (∼10 μL) of the AuNP/thiophene film was deposited on a Ramchip slide, and the sample was allowed to dry for ∼2 h. The laser power used for the acquisition of the thiophene SERS spectra is 1.3 mW, and the integration time is 500 s. All measurements were obtained using a 633 nm He−Ne laser and an Olympus 10× objective (NA = 0.25). The Raman shift was calibrated with a neon lamp, and the Raman shift accuracy was ∼0.5 cm−1. SEM-EDX and AFM Analysis of Transferred AuNP Films. The morphologies of the Au films were studied by scanning electron microscopy (SEM) using a JEOL JSM-6500F FE-SEM (Jeol Co., Japan) microscope. Using an EMS 150T ES sputter coater, a 5 nm layer of platinum was coated onto the Au film for SEM analysis. SEM energy-dispersive X-ray (SEM-EDX) measurements were performed on both AuNP and AgNP films on glass slides after coating with the 5 nm layer of platinum. The accelerating voltage used in the EDX measurement is 10 keV. An atomic force microscopy (AFM) image of the AuNP films deposited on the glass slides were also obtained by an atomic force microscope (Dimension Icon AFM with ScanAsyst, Bruker, Santa Barbara, CA) in scanasyst mode using a silicon nitride tip (scanasyst-air) attached to a cantilever with a spring constant of 0.4 N/m. SERS Application of the Transferred AgNP and AuNP Films. The AgNP and AuNP films on the thiophene were transferred by pipetting 10 μL of AgNP- or AuNP-coated thiophene solution onto a RamChip slide. The films were first air-dried for ∼2 h. After SERS spectra acquisition of the air-dried film, the AuNP and AgNP films on the Ramchip slide were heated in an oven at 120 °C for 13 h and the SERS spectra were acquired again after allowing the slide to cool to
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RESULTS AND DISCUSSION Self-Assembly of AuNPs and AgNPs onto Thiophene. Unless specified otherwise, all of the AuNP film preparation described in this work was obtained with citrate-reduced AuNPs with a particle diameter of ∼13 nm and a concentration of ∼10.7 nM (Supporting Information, Figure S1). The AuNP concentration was calculated using the peak absorbance at 520 nm and an extinction coefficient of 2.7 ×108 M−1 cm−1 for 13 nm AuNPs.40 In a typical experiment, the as-synthesized citratereduced AuNPs are mixed with neat thiophene. Vortex mixing the AuNP/thiophene mixture induces immediate AuNP accumulation onto the thiophene surfaces (Figure 1). Figure
Figure 1. (A) 1 mL of neat thiophene and (B) 2 mL of AuNP before mixing. The AuNP diameter and concentration are 13 nm and 10.7 nM, respectively. (C) AuNP/thiophene mixture right after mixing the AuNP and thiophene from vials A and B. (D−G) Snapshots taken ∼2 s, ∼10 s, ∼30 s, and 2 h after the thiophene/AuNP mixture was shaken with a vortex mixer at a speed of ∼3200 rpm for ∼1 min. (H) AgNPs self-assembled onto thiophene oil. The AgNP film was prepared by mixing 1 mL of thiophene with 2 mL of a 2-fold-concentrated solution of the as-synthesized AgNPs at a speed of ∼3200 rpm for ∼1 min.
1A,B shows thiophene and AuNP before mixing. Figure 1C shows the AuNP/thiophene mixture just after mixing the samples shown in vials A and B. A large number of small, golden-colored droplets are produced when the mixture is shaken for ∼1 min (Figure 1D,E). Once the shaking stops, the golden droplets very rapidly settle to the bottom of the glass vial and coalesce (Figure 1E,F). The bottom layer of the solution appears to be a piece of solid gold in the glass vial (Figure 1G) where the thiophene/water and the thiophene/ glass interfaces (the sides and the bottom) are all covered with a AuNP film with an intense gold hue. The entire process from AuNP/thiophene mixing to complete AuNP film formation takes less than 5 min (video 1 in the Supporting Information). AgNP film formation on thiophene oil was also observed when colloidal AgNPs in water were vortex mixed the same way with thiophene oil (Figure 1H). 11521
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Thiophene Floating Experiment. The AuNP and AgNP films are located exclusively at the glass/thiophene and water/ thiophene interfaces. This is experimentally confirmed with Raman spectroscopic measurements with AuNP/thiophene (Figure 2) and AgNP/thiophene (Figure S3 in Supporting
Figure 3. Photographs of (A, F) as-prepared AuNP and AgNP films in water in a 15 mL glass vial where ∼7.5 mL of thiophene was enclosed by AuNPs and AgNPs, respectively. (B, G) AuNP and AgNP films elevated from the bottom of the glass vial by replacing water in vials A and F, respectively, with saturated KCl solution. (C, H) AuNP and AgNP films at the thiophene/air interface are broken when the films float to the top of the aqueous layer. (D, I) After removal of ∼1.5 mL of thiophene from vials C and H through the opening at the thiophene/air interface. No AuNPs or AgNPs are observed in the removed thiophene contained in vials E and J, respectively.
Figure 2. (a) Raman spectra of neat thiophene and (b, c) Raman spectra taken by focusing the laser beam (b) through the AuNP film and (c) outside the AuNP film. The data obtained with the AgNP/ thiophene sample are very similar to those of the AuNP/thiophene mixtures (Supporting Information). The sampling positions for spectra b and c are indicated schematically in the inset. The spectra are offset and scaled for clarity. The spectral integration time for the Raman spectrum of neat thiophene and spectrum b is 5 s, and that for spectrum c is 500 s. The laser power impinging on the sample vials is 13 mW. All measurements were obtained using a 633 nm He−Ne laser and an Olympus 10× objective (NA = 0.25). The sample was prepared by vortex mixing 1 mL of 107 nM AuNPs with 3 mL of thiophene oil at a speed of ∼3200 rpm for ∼1 min.
air, the AuNP and AgNP films at the air/thiophene interface break immediately (Figure 3C,H), leaving an opening that allows us to examine whether there are AuNPs or AgNPs dispersed inside the thiophene layer (Figure 3D,I). This NP film breaking at the air/thiophene interface is driven by minimizing the total energy of the system. The Van der Waals interaction between NPs and molecular silica on the glass vial should be much stronger than that between NPs and molecules in air. To reduce the total energy of the system, the NP film originally at the air/thiophene interface goes to the wall of the glass vial. The AuNP and AgNP films at the thiophene/water and thiophene/glass interfaces remain intact after the removal of the thiophene enclosed inside the AuNP (Figure 3C,D) or AgNP film (Figure 3H,I). The total absence of AuNPs or AgNPs in the thiophene layer (Figure 3E,J) and the water layer (Figure 3D,I) indicates that NPs are completely and exclusively assembled on the water/thiophene and thiophene/glass interfaces. AuNP Film Expansion. The AuNP film on the thiophene surface is physically sturdy in that it can be “expanded”, “shrunken”, and “deformed” to accommodate the shape change of the thiophene layer. AuNP film expansion can be achieved by injecting more thiophene into the AuNP-enclosed oil (Supporting Information Figure S5), which reduces the AuNP packing density at the thiophene surface as shown by the reduced AuNP surface plasmonic resonance (SPR) absorbance (Supporting Information Figure S5). Conversely, the surface area of the AuNP film can be reduced by reducing the amount of thiophene enclosed by AuNPs. For example, after thiophene removal from vial D in Figure 3, thiophene is completely reenclosed by AuNPs and submerged in water by reducing the KCl concentration in the aqueous phase (Figure 4A). The surface area of the AuNP film in Figure 4A is significantly smaller than that in Figure 3B. AuNP film deformation is shown in Figure 4B. When the glass vial is completely filled with water, the shape of the AuNP-enclosed thiophene can be deformed by shaking the glass vial without breaking the AuNP film. This further demonstrates that the AuNP film is extremely
Information) samples. The Raman spectra taken by focusing the laser inside the AuNP- or AgNP-coated liquids (Figure 2b, Figure S3b) are identical to those obtained with neat thiophene (Figure 2a, Figure S3a), whereas the Raman spectrum taken outside the NP film contains both thiophene and water Raman features (Figure 2c, Figure S3c)). The latter spectrum allows us to estimate the thiophene/water molarity ratios of the solvents both inside and outside the AuNP or AgNP films. The solvent inside the AuNP and AgNP films is predominantly thiophene in which the thiophene concentration is at least 128 times higher than that of water, whereas the solvent outside the NP film is almost entirely water (>99.8% for AuNP and >99.9% for AgNP). This estimation is based on the thiophene/water Raman activity ratio of 1233 that is determined with the thiophene and water Raman spectrum acquired under identical experimental conditions (Figure S4 in Supporting Information). These results, together with the experiment result shown in Figure 3, indicate that the NP film is formed exclusively on the thiophene/glass and thiophene/water interfaces, completely enclosing the thiophene oil phase. The density of thiophene is 1.05 g/cm3. The AuNP- and AgNP-coated thiophene (Figure 3A,F) can be moved to the top of the glass vial by gradually replacing the aqueous solution in the glass vial with saturated KCl solution. The film remains visually intact as long as the thiophene is completely submerged in water, but the shape of the AuNP- and AgNP-coated thiophene phase changes, likely as a result of the specific gravity change in the aqueous phase (Figure 3B,G). However, once the thiophene moves to the top of the aqueous layer and the NP film at the upper thiophene/water interface is in contact with 11522
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absorbance intensity is either due to the transition from submonolayer to monolayer or from one layer to multilayers. We can clearly observe that complete thiophene coverage is formed in samples D−F in Figure 5. Because the AuNPs are exclusively located on the thiophene surfaces, this result demonstrates that the average thickness of the AuNP film can be controlled by adjusting the AuNP concentrations. Not surprisingly, there is a strong surface plasmon resonance (SPR) coupling among the AuNPs accumulated on the thiophene surfaces as indicated by a large shift in the AuNP SPR band maxima and band broadening. The lowest SPR peak wavelength of the AuNP film on the thiophene surface is ∼695 nm (Figure 5II), which is red-shifted by more than 170 nm from that of the as-synthesized AuNPs (Figure 5II, black dotted spectrum) and ∼100 nm from that of the AuNPs retained in the aqueous phase in the AuNP/thiophene mixtures (Figure 5I). This strong AuNP SPR coupling is due to the AuNP aggregates formed before or during the AuNP assembly onto the thiophene surfaces. Effect of Mixing Speed on AuNP Film Formation. The speed of vortex mixing (in rpm) the AuNP/thiophene mixture also plays a critical role in the rate of formation and morphology of the AuNP film. Without shaking, it takes more than 15 h for the AuNPs to completely clear out of the aqueous phase (Supporting Information Figure S6). In this case, AuNPs accumulate only at the curved water/thiophene interfaces (Figure 6, sample a in the photograph) instead of forming 3D complete thiophene surface coverage as in the high-speed vortex-mixed samples (Figure 6, samples b−f in the photograph).
Figure 4. Photographs of (A) thiophene completely re-enclosed by AuNPs and resubmerged in water after replacing a fraction of the KCl solution in Figure 3D with water. (B) Deformation of the AuNP film submerged in water by shaking.
flexible. Only violent shaking can break the film into small droplets of AuNP-coated thiophene; however, these small droplets rapidly coalesce once the shaking is stopped. (Supporting Information video 2). AuNP Concentration Dependence of Film Formation. The number of AuNPs accumulated onto the thiophene surface depends critically on the AuNP concentration. We investigated AuNP film formation using a series of AuNP/thiophene mixtures with AuNP concentrations of 1, 3, 5, 8, 21, and 64 nM, but the volumes of thiophene and water were kept constant (Figure 5). AuNP accumulation at the thiophene
Figure 6. (Left) UV−vis spectra of the top layer in the thiophene/ AuNP mixtures after 18 h of sample incubation. (Right) Photographs of thiophene/AuNP mixtures prepared with a vortex mixer with speed settings of ∼0, 600, ∼1178, ∼1756, ∼2334, and ∼3200 rpm for samples a−f, respectively. The samples were prepared by adding 1.5 mL of thiophene to 1.5 mL of a 10.7 nM AuNP colloidal solution and mixing for ∼1 min. The photograph was taken after the samples sat under ambient conditions for 18 h.
Figure 5. (Top) AuNP/thiophene mixtures prepared by mixing 1.5 mL of AuNP and 1.5 mL of thiophene at a speed of ∼3200 rpm for ∼1 min. The nominal AuNP concentrations from vials A−F are 1, 3, 5, 8, 21, and 64 nM, respectively. (Bottom, I) The UV−vis spectra obtained from the aqueous phase (top layer) of the solutions in vials A−F. (Bottom, II) UV−vis spectra of the AuNP films (bottom layer) of the solutions in vials A−F. The UV−vis spectra of the films were obtained from the same samples prepared in a quartz cuvette. The black dotted spectrum is obtained with 5.5 nM as-synthesized AuNPs in water.
Formation of AuNP Films Using Different Sizes of AuNPs. Besides the 13 nm citrate-reduced AuNPs described above, all of the AuNPs we have tested can form a AuNP film on the thiophene surface. These AuNPs include sodium borohydride-reduced AuNPs with diameters of ∼5 nm, the commercial tannic acid-reduced AuNPs with a nominal diameter of 10 nm, and in-house-synthesized citrate-reduced AuNPs of ∼13 and 30 nm (Supporting Information Figure S7). This manifests the general applicability of this AuNP filmformation method. AgNP Film on Thiophene. Citrate-reduced AgNPs can also self-assemble onto the thiophene surface with this method. However, unlike the AuNP film that exhibits excellent stability, the AgNP films rapidly tarnish under our experimental
surface was observed in all of the samples, and the amount of AuNPs that assemble onto the thiophene surface increases with increasing AuNP concentrations. This is indicated by the increase in the golden luster of the film as the AuNP concentration increases (Figure 5, vials A−F). Figure 5II (D−F) shows that the resonance peak wavelength changes only slightly when the AuNP concentration is larger than 3 nM, indicating that the plasmonic coupling between neighboring AuNPs is stable or the interparticle distance between AuNPs remains approximately a constant. The change in the 11523
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conditions. The AgNP films in the freshly prepared samples are uniform with a shiny silver color (Figure 7A, samples b−d, and
primarily to the thiophene reaction with the silver oxide on the as-synthesized AgNPs. The exact formation mechanism of AuNP and AgNP films on the thiophene surface is currently unclear. However, multiple forces, including mechanical and chemical interactions, should contribute to the AuNP and AgNP accumulation onto the thiophene surfaces. Vortex mixing creates shear forces that can be particularly strong at the water/oil and oil/substrate interfaces, facilitating NP aggregation and concentration to these locations,42 whereas the interparticle van der Waals forces likely contribute significantly to the formation of the NP film on the thiophene surfaces. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) studies indicate that the sulfur headgroups in thiophene can chemically interact with gold.43,44 The ability of thiophene to displace stabilizing agents such as citrate on the as-synthesized NP surfaces is important in reducing the NP electrostatic repulsions. Regardless of the exact mechanism, we believe that thiophene adsorption onto AuNPs and AgNPs precedes NP film formation. This conclusion is also supported by the subsequent SERS measurements that show that the complete removal of thiophene SERS features on NPs requires relatively lengthy heating at elevated temperature. SERS Application of the Transferred AgNP and AuNP Films. The AuNP and AgNP films formed in the glass vials can be transferred to a glass slide and stainless steel surfaces by transferring AuNP- or AgNP-coated small thiophene droplets to the solid supports. Even though the films usually disintegrate into small pieces during the transfer process, they rapidly reform at the thiophene/water interface and the thiophene/ glass surface (Figure 8A). Upon solvent (both water and thiophene) evaporation, thin AuNP and AgNP films are formed on the glass slide and stainless steel surfaces. Figure 8B shows a representative SEM image obtained with the dried AuNP film deposited on a glass slide, which showed that the large size of the AuNP film (>40 μm in one dimension) was coated on the glass slide. The AFM image of the dried film shows that the AuNP film was quite smooth in the sampled region. The largest height difference from the peak to valley of the deposited AuNPs is about 12.4 nm, which is close to the average diameter of AuNPs used for sample preparation (Supporting Information Figure S9). Importantly, even though thiophene is a highly volatile organic oil, air-drying alone cannot completely remove thiophene from the AuNP and AgNP films, as evident from the SERS spectra obtained with the air-dried films (Figure 8, spectrum a in both C and D). This is due to the strong thiophene binding affinity to AuNPs and AgNPs. The thiophene SERS spectra of the air-dried AgNP and AuNP films are drastically different from each other (Figure 8, spectra a in both C and D). The normal Raman spectrum of thiophene is very similar to that reported by Klots et al.45 The peak assignments shown in the Supporting Information (Tables S1 and S2) were based on the work by Pasterny et al. and Kupka et al. on neat thiophene.46,47Compared to that obtained with AgNPs, the thiophene SERS spectrum on AuNPs is much more similar to the thiophene normal Raman spectrum. This result indicates that the structure of thiophene on AuNPs is significantly different from that on AgNPs. The exact reason for this difference is unclear as it is currently impossible to detect the molecular structure of the molecule on the NP surfaces. One possible reason is that thiophene adsorption onto AuNPs is through direct thiophene interaction with the surface gold atoms, but thiophene is adsorbed onto AgNPs via reaction with
Figure 7. (A) Photographs of the (a) as-synthesized AgNPs and (b− d) freshly prepared AgNP/thiophene mixtures. (B) Photograph of the same set of samples in A but aged for 10 h. Samples b−d were prepared with the as-synthesized, 2-fold-concentrated, and 3-foldconcentrated citrate-reduced AgNPs, respectively. The samples were prepared by mixing 1.5 mL of AgNPs in water with 1.5 mL of thiophene in a 4 mL glass vial at a speed of ∼3200 rpm for ∼1 min. (C, D) Bright-field microscopy images of the freshly prepared AgNP film and the same film aged for 10 h, respectively. The bright-field images were acquired from AgNP/thiophene mixtures in a 3.5 mL quartz cuvette. The sample for the bright-field image was prepared by mixing 1.5 mL of the 2-fold-concentrated AgNP solution with 1.5 mL of thiophene. The images were acquired using an Olympus microscope with a 10× objective for the Raman instrument.
Figure 7C). However, the color of the silver film becomes increasingly dimmer with time after the sample preparation. After 10 h of aging, the AgNP film breaks into small flakes floating at the thiophene/water and thiophene/glass interfaces (Figure 7B, samples b−d, and Figure 7D). Importantly, the speed of AgNP film discoloration appears to be independent of the age of the as-synthesized AgNPs in that shiny AgNP films can be prepared with both freshly synthesized and 2-monthaged AgNPs, but the AgNP films are discolored after a similar period of sample incubation. Our hypothesis is that the AgNP film discoloration and disintegration is due to AgNP oxidation or the reaction of the thiophene with surface silver oxide that is known to be present on AgNP surfaces. This is in contrast to AuNPs, where the surface atoms on AuNPs are mostly neutral.33,41 It is noteworthy that a AgNP film prepared by mixing the AgNPs synthesized under ambient conditions with thiophene in a nitrogen-filled glovebox also tarnishes within a time span similar to that for AgNP films prepared under ambient conditions (Supporting Information Figure S8). This result leads us to believe that the AgNP film tarnishing is due 11524
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SERS spectrum obtained with colloidal AgNPs and AuNPs in water.50,51 This indicates the fidelity of the SERS spectra of R6G deposited on the NP films. Heat treatment induced no significant change in the morphology of the AuNP film deposited on the glass slide but induced extensive AgNP fusion in the AgNP film deposited on the glass slide in which many isolated AgNPs in the air-dried AgNP film are fused together (Supporting Information Figure S11). One likely reason for the AgNP fusion is that silver has a lower melting point than gold so that AgNP deforms more easily than AuNP at elevated temperature. Importantly, this AgNP morphological change did not eliminate the SERS activity of the AgNP films as shown by the data in Figure 8.
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CONCLUSIONS
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ASSOCIATED CONTENT
We demonstrated in this work that AuNPs and AgNPs can readily self-assemble into AuNP or AgNP films that completely cover thiophene surfaces in water. The self-assembly of the NPs occurs without using any exogenous surfactant, polymer, or cross-linker as the NP surface modifier or as the promoter. Although the AuNP film exhibits excellent stability, the AgNP film can be readily tarnished and disintegrated under ambient conditions These results, combined with a series of our recent studies of the organothiol interactions with colloidal AuNPs and AgNPs,32,33 indicate that AgNPs and AuNPs can be significantly different in their interactions with organosulfur compounds.
Figure 8. (A) AuNP-coated thiophene droplets transferred to glass substrates and stainless steel. (B) Representative SEM image of the AuNP film on the glass slide after solvent evaporation. (C, D) SERS spectra obtained with (C) the AgNP film and (D) the AuNP film transferred to a RamChip slide. Spectra a and b) were acquired with the air-dried and oven-heated (120 °C for ∼13 h) films, respectively. Spectrum c was acquired after depositing 5 μL of 5 and 10 μM R6G on the AgNP and AuNP films, respectively, after heat treatment and cooling. SERS spectra a and b were acquired with a 633 nm He−Ne laser and an Olympus 10× objective (NA = 0.25). The laser power is 1.3 mW. The spectral integration time is 500 s. The SERS spectra of R6G were acquired using a 785 nm laser. The laser powers used for the R6G SERS spectra are 0.4 and 1 mW for the AgNP and AuNP films, respectively. The spectra acquisition time for all R6G SERS spectra is 500 s. The spectra are plotted on the same scale but offset for clarity.
S Supporting Information *
Video demonstration of the process of making the AuNP films and the shape change of the AuNP film in a glass vial. UV−vis spectrum and TEM image of AuNPs. UV−vis spectrum of AgNPs. Raman spectral characterization of AgNP assembled onto thiophene oil. Thiophene content in the aqueous phase of the AuNP/thiophene and AgNP/thiophene mixture. Comparison of thiophene and water Raman activities. AuNP film expansion and effect of mixing speed on AuNP film formation. Formation of AuNP films using different sizes of AuNPs. Comparison between AgNP film prepared in the absence and presence of nitrogen. Atomic force microscopy (AFM) image of AuNP film transferred to the glass slide. Peaks detected in Raman spectrum of neat thiophene and SERS spectra of the NP films. SEM-EDX measurements of air-dried and oven-dried AuNP and AgNP films. SEM-EDX measurement of the sulfur− gold and sulfur−silver ratios for air-dried and oven-dried AuNP and AgNP films. SEM images of AuNP and AgNP films before and after heat treatment. This material is available free of charge via the Internet at http://pubs.acs.org.
surface silver oxide. Work by others and our group showed that surface silver atoms on AgNP are oxidized.32,48,49 Another possibility is that thiophene on AuNPs and AgNPs may have different susceptibilities for chemical and photochemical reactions. Previous research showed that thiophene can be photo-oxidized under ambient conditions.44 Importantly, the thiophene or thiophene derivative SERS features on the air-dried AuNP or AgNP film can be drastically reduced by oven-heating (Figure 8 spectra b in both C and D). The thiophene Raman features in both the air-dried AuNP and AgNP films become invisible after oven heating them at a temperature of 120 °C overnight (∼13 h). SEM-EDX measurements were employed to investigate the possible thiophene evaporation with oven heating. However, there is no significant change in the S/Au or S/Ag ratio in the ovendried films in comparison to those of their air-dried counterparts (Supporting Information Figure S10 and Table S3). This result indicates that the absence of the thiophene SERS feature in the oven-treated sample is not due to thiophene evaporation. One possible reason is that thiophene is decomposed on the heated AuNP and AgNP films, leaving a sulfur-containing reaction product on the NP surfaces that apparently has very low SERS activity. This hypothesis is consistent with the fact that thiophene can be oxidized on the noble metal NPs.44 Further experiments are needed to confirm this hypothesis. Nonetheless, the oven-treated AgNP and AuNP films remain SERS-active. The SERS activity of the AgNP and AuNP films was demonstrated with rhodamine 6G (R6G) as a model analyte. The R6G SERS spectra obtained with the AgNP and AuNP films are very similar to each other (Figure 8c in C and D), and they are consistent with the R6G
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by an NSF CAREER Award (CHE 1151057) and NSF funds (EPS-0903787) provided to D.Z. S.Z is grateful for support from ONR and NSF for the research. 11525
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