One-Pot Synthesis of Monodispersed Silver Nanodecahedra with

Oct 25, 2012 - Department of Life Science, National Chung Cheng University, 168 University Road, Ming-Hsiung, Chiayi County 62102, Taiwan...
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One-Pot Synthesis of Monodispersed Silver Nanodecahedra with Optimal SERS Activities Using Seedless Photo-Assisted Citrate Reduction Method Li-Chen Yang,† Yen-Shang Lai,† Chin-Ming Tsai,† Yi-Ting Kong,† Cheng-I Lee,*,‡ and Cheng-Liang Huang*,† †

Department of Applied Chemistry, National Chiayi University, 300 Sha-Fu Road, Chiayi City 60004, Taiwan Department of Life Science, National Chung Cheng University, 168 University Road, Ming-Hsiung, Chiayi County 62102, Taiwan

‡

S Supporting Information *

ABSTRACT: This article presents a mechanistic study of silver nanodecahedra prepared by the method of photoassisted sodium citrate reduction under irradiation of blue light-emitting diodes (LEDs). This synthesis of silver nanodecahedra can be easily reproduced in the absence of silver seeds and can be completed in one-pot. The data suggest a combination of two processes including gradual growth from multiple-twinned particles and subsequent plasmonmediated crystal growth. More than 85% of as-prepared silver nanoparticles are decahedra with edge lengths of 43.5 ± 5.2 nm. In addition, the as-prepared silver colloids exhibit a spectroscopic enhancement in comparison with spherical silver nanoparticle colloids and silver nanoprism colloids in the measurement of surface enhanced Raman spectroscopy (SERS) spectra of Rhodamin 6G, and with the decahedral silver colloids synthesized in the presence of PVP for detecting SERS signal of Rhodamin 6G. Overall, this photoassisted citrate reduction process is simple, and highly reproducible. As-prepared silver nanodecahedra are more stable and provide optimal SERS signal than those synthesized using commonly used plasmon-mediated photochemical methods.

1. INTRODUCTION Silver nanoparticles (NPs) have been applied in the fields of biosensing,1−5 and optoelectronic devices,6−8 and have been used as active substrates for surface-enhanced Raman spectroscopy (SERS).9−21 Most of these applications are based on their optical properties, which originate from light absorption and scattering by surface plasmon resonance (SPR).22,23 The SPR bands of silver nanoparticles are typically located in the visible and near-infrared region, and are strongly dependent on the size, shape, composition, crystallinity, interparticle spacing, and the local dielectric environment.24,25 It is believed that controlling the shape of the silver NPs is one of the most efficient methods to obtain NPs that exhibit the desired SPR wavelength.26−29 A number of successful methods have been developed to successfully synthesize silver nanoparticles in the solution phase having particular shapes, such as tetrahedrons,30 cubes,31−33 bars, 34 bipyramids, 35 octahedrons, 36 decahedrons, 37−42 prisms,43−47 hexagonal plates,48 disks,49−52 rods,53,54 and wires.55−58 These methods can be classified into two main categories: chemical reduction and photochemical methods. Common chemical reduction methods include citrate reduction, silver mirror reaction, polyol process, seed-mediated growth. Photochemical methods, such as plasmon-mediated synthesis,27,59 provide a relatively effective approach to study the mechanism of formation because these reactions can be terminated immediately by removal of the light irradiation.47 Light-emitting diodes (LEDs) are more efficient than tradi© 2012 American Chemical Society

tional lamps as a result of their relatively narrow-band emission, high emission intensity, and low energy consumption. Therefore, LEDs have been recently used as the source of excitation light in plasmon-mediated reactions to synthesize silver NPs of particular shapes.39,40 In this study, silver nanoparticles with excellent crystalline structure were prepared by photoassisted sodium citrate reduction under the irradiation of blue-LEDs. This method is relatively facile; the experimental results can easily be reproduced since no silver seeds are required prior to light irradiation. Because the size and shape distributions as well as the crystalline structures are highly dependent on the operational process, the lack of the need for silver seeding removes the dependence of the structure of the synthesized silver NPs on the structure of extrinsic silver seeds. The asprepared silver nanoparticles are relatively narrow-sizedistributed. More than 85% of the nanoparticles are decahedra with edge lengths of 43.5 ± 5.2 nm. The remaining nanoparticles are nanoplates and tetrahedrons. Compared to the silver colloids synthesized by thermal citrate reduction methods, our as-prepared silver colloids exhibited superior size and morphology distribution, and easily avoided aggregation in the ambient environment for more than 6 months. Furthermore, the stability of the as-prepared silver decahedra Received: June 26, 2012 Revised: October 24, 2012 Published: October 25, 2012 24292

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is superior to that of the silver decahedra synthesized by the method of plasmon-mediated shape conversion from spherical seeds under the irradiation of blue LEDs at 0 °C.

2. EXPERIMENTAL SECTION 2.1. Materials. Silver nitrate, sodium citrate, sodium borohydride, KBr, 16-mercaptohexadecanoic (MHA), and PVP (Mw = 40, 000) were all purchased from Sigma-Aldrich. These reagents were not further purified before use. Milli-Q grade water (>18 MΩ) was used in all of the experiments. 2.2. Instrumentation. Joel JEM-2100 transmission electron microscopy (TEM) was used to collect TEM images of each sample and was operated at 100 KV or 200 KV. Before analysis by TEM, aliquots of silver colloids were dripped onto a carbon-coated copper grid and dried in air at room temperature. All UV−vis extinction spectra were recorded at 25 °C on a Hitachi U-2800 spectrophotometer using a quartz cuvette with an optical path length of 10 mm. Powder X-ray diffraction (XRD) was performed on a Bruker D8 Discover X-ray diffractometer with a Cu Kα radiation source, and the sample was deposited onto a glass slide. 2.3. Colloid Preparation. The silver nanoparticles were prepared by a photoassisted citrate reduction method under the irradiation of blue LEDs. In a typical synthesis, 1 mL of sodium citrate (4.5 × 10−1 M) and 1 mL of silver nitrate (1.0 × 10−2 M) were mixed with 98.0 mL of pure water. The mixture was subsequently irradiated with 24 blue LEDs (λmax = 460 ± 12 nm, average power ≈ 100 mW/cm2) assembled on a cylindered reaction pot. Silver nanoparticle colloids with more than 85% nanodecahedra were obtained after 90 min of irradiation. The silver colloids exhibited an SPR band at 499 nm. For the temperature-controlled experiments, the vial with the mixture was placed in a tank connected with a temperature-controlled water circulating system and was irradiated under the blue LEDs as described above. Figure S1 of the Supporting Information shows the setup of the temperature-controlled light-irradiation system. The irradiation time for a reaction was 48 h and 60 min for 20 and 80 °C, respectively. 2.4. SERS Measurement. The SERS spectra of crystal violet (CV) and Rhodamin 6G (R6G) were recorded in several types of silver colloids, including the as-prepared silver nanoparticle colloid, decahedral silver nanoparticle colloid synthesized at 0 °C using a seed-mediated method, silver nanoprism colloid, silver nanoparticle colloid synthesized by the thermal citrate reduction method, and silver nanodecahedral colloid synthesized in the presence of PVP using the method developed by Kitaev et al., in the presence or absence of KBr to compare the enhancement capability of these SERS substrates. For a typical measurement, a solution of R6G (10−8 M, 0.9 mL) or a solution of CV (10−6 M, 0.9 mL) was added to 1 mL of each silver colloid. KBr (1 M, 0.1 mL) was added after 30 min of mixing. Prepared samples were placed in a quartz cuvette and excited by a 30 mW laser beam at 532 nm. The typical acquisition time of a SERS spectrum was 40 s.

Figure 1. Silver nanodecahedra synthesized by photoassisted citrate reduction method. (a) TEM images and (b) UV−vis extinction spectrum of the as-prepared silver nanoparticles.

the Supporting Information. In comparison to the products synthesized by thermal sodium citrate reduction, our asprepared silver nanoparticles were significantly monodispersed in size.12−19 More than 85% were silver decahedra with the edge length 43.5 ± 5.2 nm, and with remainder being tetrahedrons, silver nanoplates, and those assumed to be other types of polyhedrons. The UV−vis spectrum illustrated a strong peak centered at 499 nm, and a weak and sharp peak centered at 408 nm, as shown in part b of Figure 1. According to the assignments in the previous studies, the peak at 499 nm was attributed to the longitudinal dipolar SPR modes of the silver decahedra with edge length of 43.5 ± 5.2 nm.38,39 However, the assignment for the peak at 408 nm has not been properly addressed in the related literature.38 The citrateprotected spherical silver nanoparticles also exhibit a dipolar SPR band at ca. 400 nm; however, this peak usually has a broader bandwidth (approximately 70−90 nm) than that of the peak at 408 (approximately 42 nm), as shown in part b of Figure 1. Moreover, similar to silver nanoprisms, the asprepared silver nanodecahedra are also sculpted by the halide

3. RESULTS AND DISCUSSION 3.1. Formation of Silver Nanodecahedra. Figure 1 shows the TEM images of the silver nanoparticles and the corresponding UV−vis extinction spectrum, respectively. The products were synthesized by our photoassisted citrate reduction method in 90 min. The size distributions of silver nanoparticles at various reaction times are listed in Table S1 of 24293

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ions.29,60−62 Part a of Figure 2 shows the TEM images of the asprepared silver nanodecahedra in the presence of 0.01 M KBr

in part b of Figure 3 with an inserted plot showing the change of the peak at 402 nm in the early stage. Obviously, the peak at 402 nm grows significantly in the early stage of the synthesis within 8 min. Because this peak is correlated to the typical SPR band of citrate-protected spherical silver nanoparticles, this result implies that silver cations can be reduced by citrate ions under the irradiation of intense blue light. Although the reduction of silver cations by citrate alone is thermodynamically allowed,63 the reaction rate is slow at room temperature due to the existence of a relatively high energy barrier that must be overcome.47 Therefore, we tentatively proposed that the silver nanoparticles during the early stage were generated through a photoassisted citrate reduction process. This photoassisted citrate reduction process differs from the plasmon-mediated process47 because this method does not require silver seeds at the initial stage. In contrast, the silver seeds (usually formed from the silver cations reduced by NaBH4) in the plasmonmediated process can serve as a photocatalyst to interact with the incident visible light to generate the hot electrons and hot holes for the following redox reactions of silver cations and citrate ions.47 In the beginning stage of the photoassisted citrate reduction process, a thermal reduction process may be involved,47 or some silver clusters (or silver-citrate complex) may interact with the incident light to initiate the formation of seeds in the presence of sodium citrate. Figure S2 of the Supporting Information shows the UV−vis spectrum of the mixture of 5 × 10−3 M AgNO3 and 2.25 × 10−1 M sodium citrate. Absorption at 456 nm is quite weak at ∼6 × 10−3, but this sole absorption is very critical. We believe that the absorbed light activates the citrate ion so as to overcome the reaction barrier to reduce the silver ions and to form the silver nanoparticles in the initial stage. As shown in part b of Figure 3, the intensity of the SPR band at 402 nm increased from 0 to 0.1, 0.5, and 1.05 after 4, 6, and 8 min of reaction, respectively. The time-course spectra of the solutions in the first ten minutes are compared preciously in Figure S3 of the Supporting Information. The absorption at ∼400 nm started increasing in the second minute and reached the maximum absorption at the ninth minute. This phenomenon can be explained by the combination of two mechanisms in this synthesis. The primary process is photoassisted initiation of seeds in the first 4 min. Therefore, the SPR intensity increased slightly to 0.1. Once the seeds are effectively formed, plasmon-mediated crystal growth strongly dominates and increases the SPR signal to 0.5 and 1.05 rapidly, whereas the photoassisted reaction may continue concurrently. Because the morphologies of the final structures were determined by the crystalline structures of the seeds,64 we collected the TEM images of the nanostructures as early as possible in the photochemical reaction. As shown in part c of Figure 3, most of the quasi-spherical seeds are small, and a number of multiple-twinned particles (MTPs) of ca. 20 nm in size can be observed (inserted picture in part c of Figure 3) in the early stage (6 min). Part d of Figure 3 (10 min) shows that most of the seeds are MTPs within a edge-length range of 12− 25 nm, and several decahedra (ca. 30 nm in edge-length) can also be observed. This observation is consistent with the appearance of a shoulder peak at approximately 485 nm corresponding to the longitudinal dipolar resonance of smaller decahedra in spectrum D (10 min) of part a of Figure 3. Parts e−g of Figure 3 demonstrate the TEM images of the asprepared silver colloids at 20, 50, and 90 min, respectively. These images show that most of the quasi-spherical seeds

Figure 2. Sculpting effect of KBr on silver nanodecahedra. (a) TEM images of the as-prepared silver colloids in the presence of 10−2 M KBr for 35 min. (b) Time-course spectra of silver nanodecahedra in the presence of 0.01 M KBr.

for 35 min. These images demonstrate that the corners and edges of nanodecahedra were etched by the bromide ions. Part b of Figure 2 demonstrates the time-course extinction spectra of as-prepared silver nanodecahedra in the presence of 0.01 M KBr. After the addition of KBr for 35 min, the peak at 408 nm and the peak at 499 nm blue-shifted to 385 and 443 nm, respectively. The phenomenon in which these two SPR bands blue-shifted simultaneously implies that these major peaks both originate from silver nanodecahedra. Consequently, we assigned the peak at 408 nm to the transverse dipolar SPR modes of the silver nanodecahedra. Part a of Figure 3 illustrates the spectra of silver colloids synthesized at various reaction times with the corresponding TEM images of these silver colloids shown in parts c−i of Figure 3. Because the silver nanoparticles may be unstable in the ambient environment, 10−5 M MHA was added to the colloids prior to the TEM measurement to protect the silver NPs from possible surface etching through the formation of a self-assembled monolayer (SAM) on the surface.29 The intensity of SPR bands at 402 and 499 nm in the timecourse spectra illustrated in part a of Figure 3 was summarized 24294

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Figure 3. Photoassisted citrate reduction of silver colloids. (a) The spectra of silver colloids synthesized at various reaction times. (b) The temporal evolution of SPR bands at 402 and 499 nm with the inserted plot showing the change of the intensity of the peak at 402 nm in the early stage. TEM images of the silver colloids synthesized at (c) 6, (d) 10, (e) 20, (f) 50, (g) 90 min, (h) 9 h, and (i) 16 h.

illustrates the selected area electron diffraction (SAED) pattern, along the [110] direction, of our silver nanodecahedra. According to the assignments of gold nanodecahedra,66 the three innermost circular spots of this 10-fold symmetric image were identified as {111}, {200}, and {220}. 3.2. Effect of Bath Temperature. A previous study indicated that the photochemical reaction leads to a faceselective silver deposition, whereas the thermal reaction leads to a random growth of nanoparticles.43−47 We investigated whether shape control can be achieved by regulating the experimental conditions, such as the irradiation intensity, irradiation time, and the reaction temperature. Previously, we found that silver nanoprisms were the main products when the reaction was conducted at a high bath temperature, whereas silver decahedra were the main products when the reaction was done at a low bath temperature in the plasmon-mediated process under irradiation of blue LEDs.42 Therefore, the reaction temperature may influence the morphologies of the products in this photoassisted citrate reduction process. In contrast to the reactions resulting in the silver colloids shown in Figures 1−3, the following photochemical reactions were conducted under temperature control. In the temperaturecontrolled experiments, the temperature of the sample solution under irradiation of intense blue LEDs was increased rapidly from 20 to 85 °C, and remained constant at 85 °C after 100 min. Figure S4 of the Supporting Information shows the temperature of sample solution varied with irradiation time.

transform into decahedral silver nanoparticles gradually corresponding to the observation of the decrease of the plasmon band at 402 nm and the increase of the plasmon band at 499 nm in part b of Figure 3. In the previous studies, three possible growth processes to form silver nanodecahedra were proposed as follows: (1) edge-selective particle fusion from five tetrahedra;37 (2) one-by-one stepwise growth from tetrahedral units;41 and (3) gradual growth from MTP seeds.39 Because few tetrahedral silver NPs were observed in the TEM images, our observation indicates that the main channel in our synthetic route occurred through case 3: gradual growth from MTP seeds. Parts h and i of Figure 3 show the TEM images of the asprepared silver colloids at 9 and 16 h, respectively. The images show that most of the edges of the silver nanodecahedra were irregularly etched; and, therefore, exhibited sawtoothed structures. This photodriven etching, resulting from the overirradiation under the intense blue LEDs, differed considerably from the bromide-driven sculpting because bromide-driven sculpting leads to the formation of smooth round nanostructures (shown in part b of Figure 2). As shown in the spectra H and I in part a of Figure 3, the sawtoothed silver nanodecahedra have red-shifted SPR bands compared to the regular ones (spectrum G in part a of Figure 3). This redshift may result from the field retardation effect65 on the nonsmooth surface because the local field increases in strength on uneven surfaces, which slows electron oscillation. Figure 4 24295

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to the formation of MTPs, which act as the seeds for decahedra, and that the temperature in the later growth stage influences only the reaction rate with no effect on the morphologies of nanoparticles. This observation is consistent with a previous study in that the morphologies of final products were determined by the crystalline structures of the seeds.64 The monodispersed decahedral silver NPs are similar in reactions with and without temperature control at 20 °C. However, the synthesis without temperature control is both simple and faster than the reaction conducted at 20 °C. 3.3. Stability and the Crystalline Structure of AsPrepared Silver Decahedra. Several research groups have successfully synthesized silver nanodecahedra using the photochemical methods.38−40,42 Among these methods, our group synthesized the silver nanodecahedra by irradiating the citrateprotected quasi-spherical silver NP seeds with blue LEDs in the absence of PVP.42 However, we found that the silver nanodecahedra can only be produced at a low bath temperature and that the major products become silver nanoprisms at a higher bath temperature using the seed-mediated process. Furthermore, the silver nanodecahedra synthesized by this method undergo truncation in the corners and become more round after the photochemical reactions. Part a of Figure S5 of the Supporting Information illustrates the TEM images of silver nanodecahedra collected 40 min after the synthesis. These silver nanodecahedra were synthesized using the seed-mediated method controlled at 0 °C. The UV−vis spectra of seedmediated synthesized nanodecahedra collected immediately and 40 min after the synthesis are compared in part b of Figure S5 of the Supporting Information. Obviously, these silver nanodecahedra were losing their shape. In contrast to the seedmediated method, the silver nanodecahedra synthesized in this study are more stable and maintained their shape for more than three months. Part a of Figure S6 of the Supporting Information shows the TEM images and UV−vis spectra of as-prepared silver nanodecahedra three months after synthesis. The UV−vis spectra of these silver nanodecahedra remained the same even after three month of storage as compared in part b of Figure S6 of the Supporting Information. Although the TEM images revealed that the silver NPs by these two methods have almost the same morphology and size distribution, the instability of the NPs synthesized by the seed-mediated method at a low temperature indicates that these two NPs may differ in their crystalline structures. Figure 6 illustrates the XRD spectra of silver nanodecahedra synthesized by photoassisted citrate reduction method and seed-mediated method. The spectral features of these two nanoparticles were similar; however, the bandwidths of the peaks in the spectra differed. Both of the (111) peaks in these two spectra can be fitted with the Lorentzian functions. The peak widths of the silver decahedra synthesized by the photoassisted citrate reduction method and of those synthesized by the seed-mediated method are 0.39° and 0.59°, respectively. The grain sizes of silver decahedra synthesized by the photoassisted citrate reduction method and the seedmediated method can be estimated as 22 ± 2 nm and 14 ± 2 nm respectively using Scherrer equation. The NPs have almost the same size, but differ in grain sizes, which implies that the NPs synthesized by the seed-mediated method have more internal defects than the NPs synthesized by the photoassisted citrate reduction method. We believe that these defects in the NPs cause instability of the silver nanodecahedra in ambient surrounding.

Figure 4. Selected area of electron diffraction pattern of silver nanodecahedra. (a) Simulated electron diffraction pattern with three axis along the [110] direction. (b) Schematic [110] direction diffraction pattern for FCC silver. (c) Superimposed diffraction pattern rotated with respect to each other. The 3-fold symmetry electron diffraction patter of (a) is generated.

Figure 5 shows the TEM images of silver colloids synthesized at 20 °C for 96 h and 80 °C for 60 min, respectively. The

Figure 5. TEM images of silver colloids synthesized with temperature control (a) at 20 °C for 96 h and (b) at 80 °C for 60 min.

products of the silver colloids synthesized at 80 °C are monodispersed in size (the dimension is 62.2 ± 11.7 nm as measured from 352 nanoparticles); however, they are diverse in morphologies with the presence of a number of polyhedrons and nanoplates. The major products of the silver colloids synthesized at 20 °C are decahedra (∼80%), and the minor products are tetrahedrons (∼10%) and plates (∼10%). The synthesis without temperature control has similar product distribution but requires a shorter reaction time compared to the synthesis conducted at room temperature. These results imply that the early growth stage at a lower temperature leads 24296

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Figure 6. XRD spectra of silver decahedra synthesized by (A) photoassisted citrate reduction and (B) seed-mediated method.

3.4. Application for SERS Measurement. The previous study demonstrated that the silver nanoparticles synthesized by the thermal citrate reduction method have a high-factor of SERS enhancement.20,21 Therefore, it is interesting to compare the surface enhancement factor between as-prepared silver nanoparticles and the silver nanoparticles prepared by the thermal reduction method. Part a of Figure 7 shows the SERS spectra of CV in the silver colloids synthesized by citrate reduction and photoassisted citrate reduction methods. The SERS signals of CV in the photoassisted citrate-reduction silver colloids (abbreviated as PCR-Ag colloids) have almost the same intensities as in the thermal citrate-reduction silver colloids (abbreviated as TCR-Ag colloids). The obscure difference is that the relative intensity of 442 cm−1 band to 1620 cm−1 band is larger in PCR-Ag colloids than in TCR-Ag colloids. This result may be explained by the relationship between the enhancement factor and the frequencies of the incident laser, Raman scattering light, and SPR band of the silver NPs. Without considering the polarization of laser and the surface selection rules, the enhancement factor can be represented by eq 1: EF ∝

Figure 7. (a) SERS spectra of CV in the silver colloids synthesized by citrate reduction and photoassisted citrate reduction methods. (b) SERS spectra of 10−6 M CV in the (A) as-prepared silver NP, (B) silver nanoprism, and (C) the quasi-spherical NP colloidal solutions under the excitation of 532 nm laser.

|E Loc,A (ωP,LF)|2

|E Loc(ωL)|2 |E Loc(ωR )|2 |E Inc|2

|E Inc|2

|E Loc,A (ωP,HF)|2

(1)

where EF, EInc., ELoc(ωL) and ELoc(ωR) represent the enhancement factor, electrical fields of incident laser, enhanced local electrical fields through the interactions between incident light, and the scattering light with the SPR of metallic NPs, respectively. Van Duyne et al. performed wavelength-scanning SERS experiments and found that the enhancement factor reached a maximum when the wavelength of the SPR of NPs is equal to the average value of the wavelengths of incident light and the Raman scattering light.67 Both the SPR bands of PCRAg colloids and TCR-Ag colloids, 500 and 430 nm, respectively are not in the range of incident laser light (532 nm) and Raman scattering light (544.8 nm for 442 cm−1 mode, 582.2 nm for 1620 cm−1 mode). However, because the SPR band of PCR-Ag colloids is nearer to the wavelengths of the laser and Raman scattering light, the ratio of the enhanced local field of Raman scatter light of the low frequency mode and high frequency mode in PCR-Ag colloids is larger than this ratio in TCR-Ag colloids. This statement can be represented simply by eq 2:

>

|E Loc,C(ωT,LF)|2 |E Loc,C(ωT,HF)|2

(2)

where ELoc,P(ωR,LF), ELoc,P(ωR,HF) represent the enhanced local field of Raman scatter light of the low frequency mode and high frequency mode in PCR-Ag colloids, respectively. ELoc,T(ωR,LF) and ELoc,T(ωR,HF) represent the enhanced local field of Raman scatter light of the low frequency mode and high frequency mode in TCR-Ag colloids, respectively. Therefore, our observation results from the statement in eq 2. Part b of Figure 7 shows the SERS spectra of 10−6 M CV in the PCR-Ag, silver nanoprism, and the quasi-spherical NP colloidal solutions under the excitation of 532 nm laser. The SERS signals in the PCR-Ag are stronger than those in the silver nanoprism colloids and quasi-spherical silver colloids by a factor of 4. The reason that the as-prepared silver NP colloids are superior to the other two may result from the fact that the major SPR band of the as-prepared silver colloids (499 nm) are closer to the laser wavelength (532 nm) than the SPR bands of the other two colloids (approximately at 650 and 400 nm). The 24297

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D in part a of Figure 8. The effect of KBr is consistent with our previous study on nanoparticle colloids that the bromide ions can sculpture the silver nanoplates and induce the adsorption of dye molecules onto the surface of nanoparticles.61,68 These results indicate that PVP may have superior affinity compared to R6G for attaching to the silver NP surface. The stronger binding of PVP onto the silver NP surface compared to R6G can easily inhibit adsorption of R6G molecules on the surface of silver NPs. Consequently, PVP decreases the intensity of Raman signals and increases the fluorescence background. Part b of Figure 8 shows the SERS spectra of 10−6 M CV in the PCR-Ag and PVP-PCR-Ag colloidal solutions under the excitation of 532 nm laser. These two spectra show that the PVP-PCR-Ag colloids have slightly weaker SERS activity for CV in comparison to the PCR-Ag colloids. The reduction of CV SERS signal is due to the cover of CV molecules on the PVP coating increasing the distance to the surface. On the basis of these observations, it may be difficult to use the silver NPs synthesized in the presence of PVP as SERS substrates for specific types of probe molecules. In brief, the as-prepared decahedral silver NP colloids, synthesized in the absence of PVP, have excellent enhancement capability and fewer limitations for SERS applications.

proximity of the SPR band and the laser beam wavelength results in superior field enhancement. Kitaev et al. have successfully synthesized the monodisperse silver nanodecahedral colloids by the seed-mediated photochemical reaction in the presence of PVP and L-arginine. For simplicity and clarity in the following discussion, the nanodecahedral silver colloids are abbreviated as PVP-PCR-Ag colloids. Part a of Figure 8 shows fluorescence or SERS spectra

4. CONCLUSIONS This study successfully developed a seedless photochemical method. The present photoassisted citrate reduction process can be conducted in one-pot to synthesize monodispersed decahedral silver nanoparticles under the irradiation of blueLEDs as identified by assignment of SPR bands, TEM and SAED images. Our results suggested that this synthesis is a combination of two processes including initial formation of multiple-twinned particles serving as seeds for decahedra and subsequent plasmon-mediated crystal growth. This method is simple, and highly reproducible. The XRD spectra indicate that the grain sizes of the silver nanodecahedra synthesized by this photoassisted citrate reduction method are larger than those made by commonly used plasmon-mediated photochemical processes indicating the high stability of as-prepared silver nanodecahedra. The as-prepared silver nanodecahedral colloids have a comparable SERS enhancement factor to the silver colloids synthesized by the thermal citrate reduction method. However, the as-prepared silver colloids are significantly more homogeneous in size and morphology than those synthesized by the thermal citrate reduction method. In addition, when compared to the silver nanodecahedral colloids synthesized by the plasmon-mediated photochemical reaction in the presence of PVP, the as-prepared silver colloids of this study, which were synthesized in the absence of PVP, have relatively approachable surfaces and therefore, can provide a more suitable SERS active substrate for a number of specific probe molecules, such as R6G. SERS is an important application of nanoparticles. Because the stability of the silver particles is an important property in their utility as an active substrate, this seedless photoassisted citrate reduction is valuable for further application in optical detections.

Figure 8. (a) Fluorescence or SERS spectra of 5 × 10−9 M R6G, (A) in the absence of silver colloids, in the (B) PCR-Ag colloidal solution, (C) PVP-PCR-Ag colloidal solution, and (D) PCR-Ag colloidal solution added with 10−4 M PVP under the excitation of 532 nm laser in the presence of 0.05 M KBr. (b) SERS spectra of 10−6 M CV in the PCR-Ag and PVP-PCR-Ag colloidal solutions under the excitation of 532 nm laser.

of 5 × 10−9 M R6G in the absence of silver colloids (A), in the PCR-Ag colloidal solution (B), PVP-PCR-Ag colloidal solution (C), and PCR-Ag colloidal solution with 10−4 M PVP added (D) under the excitation of 532 nm laser in the presence of 0.05 M KBr. The fluorescence background is so strong that the Raman signal cannot be detected in the spectrum of R6G in PVP-PCR-Ag colloidal solution. However, the SERS signals of R6G are pronounced without considerable fluorescence interference in the PCR-Ag colloids. The PCR-Ag colloids containing 10−4 M PVP prior to the addition of R6G and KBr also exhibit strong fluorescence background shown as spectrum

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. 24298

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-I.L.), [email protected]. tw (C.-L.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of Taiwan (NSC 100-2113-M-415 −005 -MY2 and NSC 1012321-B-194-001). The authors would like to thank Ms. WanChing Shieh, Ms. Bi-Hua Su, and Prof. Ming-Jen Lee for measuring the TEM images; and are grateful to Professors Wenlung Chen, Chien-Chuang Cheng, and Long-Liu Lin for their helpful discussions. We also thank Dr. Raymond Chung for editing the manuscript.

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