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Pt-Pd Floating Nano-Arrays Templated on Pluronic F127 Micelles as Effective Surface-Enhanced Raman Scattering Sensors Fang-ching Chang, Yen-Cheng Li, Ren-Jye Wu, and Chun-Hua Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00434 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Applied Nano Materials

Pt-Pd Floating Nano-Arrays Templated on Pluronic F127 Micelles as Effective Surface-Enhanced Raman Scattering Sensors

Fang-Ching Chang,† Yen-Cheng Li,‡ Ren-Jye Wu,‡ and Chun-Hua Chen*† † Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-Hsueh Rd., Hsin-Chu, 30010 Taiwan, ROC. ‡ Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsin-Chu, 31040 Taiwan, ROC.

Abstract An innovative concept of floating nano-arrays (FNAs) as promising new-generation surface-enhanced Raman scattering (SERS) sensors was proposed for the first time and has been successfully realized via a facile tri-block-copolymer assisted chemical route. At least two categories of the complicated highly-uniform Pt-Pd FNAs with a fairly small assembled diameter of less than 100 nm respectively consisted of quasi-orderly assembled spherical and petal-like nanocrystals with unique radiallydistributed compositions were evidently classified and reproducibly fabricated. The synthesized Pt-Pd FNAs were found to exhibit unusual strong surface plasmon resonance (SPR) bands in the visible region, and have been successfully demonstrated as practical SERS sensors with remarkably low analytical limits of detection.

Keywords: three-dimensional, tri-block-copolymer, quasi-ordered, nano-array, SAXS, SERS

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Introduction Surface-enhanced Raman scattering (SERS), a metal-nanostructure assisted spectroscopic methodology that allows rapid detection of extremely low analyte concentration through Raman signal amplification, has been attracting attention and is being investigated for the development of promising molecular sensors.1-10 The crucial challenges facing the optimization of highly-sensitive and, more importantly, quantitative SERS sensing is the functional design and reproducible fabrication of specified SERS sensors. The sensors are comprised of uniform plasmonic nanocrystals, with precisely controlled arrangements, for creating consistent, localized and dense electromagnetic fields (hot spots) for activating SERS. Hot spots in colloidal metallic nanocrystals, the most common and simplest wet SERS sensors, are randomly distributed around the nanocrystal aggregations.11-16 This means that only extremely limited target analysis is allowed for the SERS reaction. In contrast to the colloidal nanocrystals, or even the randomly-stacked nanocrystal films, the orderly patterned two-dimensional (2D) nanostructured arrays, typically fabricated by complicated bottom-up approaches, are expected to generate consistently-controlled and high-density periodical hot spots between each neighboring nanocrystal. This will enable the coverage of many more target molecules and is advantageous not only for significantly enhancing Raman signals but for achieving the desired quantitative analyses.17-24 However, owing to inadequate contact between the analytes and nanocrystal, the reproducibility of SERS sensing remains a challenge for practical applications, even with precisely fabricated SERS-active arrays. To improve the reliability of SERS sensors, substrates with the set-off effect for concentrating analytes and coupling electromagnetic fields for boosting Raman signals is the most promising design to date. In this work, we propose and have successfully realized a promising design concept for fabricating advanced SERS sensors. The concept originated from the idea that it is possible to fold 2D monolayered SERS arrays, typically constructed on solid substrates, into three dimensional multilayered ones. This approach might potentially create additional hot spots between neighboring layers,


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to further enhance the targeting SERS signals. After considering the issue of removing rigid and nontransparent substrates from the 2D arrays, the concept was further extended to the construction of a very unique SERS sensor. This novel sensor includes three-dimensional floating nano-arrays (FNAs), comprised of shape-controlled nanocrystals that are supported by a soft, spherical self-assembled block-copolymer scaffold. The porous amphiphilic-polymer frame acts not only as the soft template for growing and assembling the quasi-ordered primitive nanoparticles (p-NPs) in the FNAs, but more functionally, it might absorb and concentrate targeting analytes via surface charge or polarity. The trapped targeting analytes in the optical-transparent FNAs could be excited by lasers. Their Raman signals would then be significantly magnified by the surrounding high-density coupling electromagnetic fields that originated from the quasi-ordered p-NPs. This approach may potentially overcome the lack of reproducibility and sensitivity of SERS sensing. The schematic diagram of the key concept is represented in Figure 1. To realize the novel design of FNAs, an amphiphilic tri-block copolymer (F127, Pluronic F127, (PEO106-PPO70-PEO106)) which has been demonstrated not only the ability to serve as a growth-directing agent in the presence of external reductants, such as ascorbic acid for synthesizing branched noble metallic nanostructures (e.g. Pt, Pd, and Au).25-33 More importantly, it has demonstrated the ability to spontaneously assemble a variety of very special periodic threedimensional packing structures over a wide concentration range, which is higher than its critical micelle concentration.34-35 The tri-block copolymer was considered as one of the preeminent soft templates for providing required reductant activation, directing growth and limiting the dimension of micro-reactors for target metallic precursors. In the present work, Pt4+ and Pd2+ ions showing the desired reduction rate in the aqueous solution of F127 were selected as the base components for building the designated FNAs. In fact, Ag and Au, the most commonly used elements for fabricating surface plasmon resonance (SPR) or SERS sensors 2, 5-6, 10, 12-13, were also applied to the developed F127 soft-template assisted synthesis with a very wide array of parameters for comparison purpose. However, due to the extremely high



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reducing rate of both Ag+ and Au4+ ions, the successful synthesis of Ag-Au or Au-Ag FNAs could not be achieved.

Results and discussion In this work, two characteristic types of Pt-Pd FNAs, namely, can be categorized according to the architecture of p-NPs in the synthesized Pt-Pd FNAs. Type A and Type B are comprised of selfassembled spherical and petal-like building blocks, respectively, and can be clearly classified as shown in Fig. 2 and Fig. S1. The transmission electron microscopy (TEM) images (Fig. 3(a)-1 and 3(a)-2) demonstrate remarkable uniformity in both the assembling size and morphology for the present Pt-Pd FNAs. This result indicates the utilization of highly consistent micro-reactors achieved through the face-centered-cubic (FCC) packed F127 micelles.35 This approach is uniquely beneficial not only for creating extremely uniform and isolated Pt-Pd FNAs, which are essential for the realization of reproducible SERS analyses but potentially enabling the tuning of the assembled architectures. Figures 3(b)-1 and 3(b)-2 show the TEM and corresponding high angle annular dark field (HAADF) images of Type A Pt-Pd FNAs, respectively. The clear granular contours and image contrast provide evidence that the Type A Pt-Pd FNAs were constructed by numerous irregularly-packed tiny nanoparticles, of about 3 nm, with a very low assembling density. This suggests the formation of a sponge- or filter-like architecture for catching target analytes that are passing through. The selectedarea electron diffraction (SAED) patterns, and the lattice fringes of Pd(111) and Pt(111) found in the HRTEM images (Fig. S2), also confirmed the absence of preferential orientation between these tiny nanoparticles in Type A. The energy dispersive X-ray spectroscopy (EDX) mapping (Fig. 3(b)-3) revealed an interesting elemental distribution regarding the formation of a Pd-rich core and Pt-rich shell. This distribution should originate from the higher redox potential of Pd, and an appropriate difference in reduction rate between Pd and Pt precursors in FCC packed F127 micelles. The EDX elemental line scans across a few FNA nanoassemblies (Fig. S3) quantitatively confirmed the radial


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gradient of the Pd:Pt ratio. The overall atomic ratio of Pd:Pt found for Type A was approximately 60:40. In addition, the simultaneous presence of carbon over the whole FNA (Fig. 3(b)-3) clarified the crucial role of soft F127 templates for gathering the tiny Pd and Pt nanoparticles. The Pt-Pd FNAs (Figs 3(c)-(e)) identified as Type B consisted of radially arranged petal-like building blocks. The blocks could be transferred from Type A, i.e. Type B-1, or be directly prepared by adjusting the weight percentage of metal precursor solution in the mixture gel, i.e. Type B-2 and Type B-3. Type B-1 (Fig. 3(c)) was directly converted from Type A by providing a long reaction time about two months. This approach led to distinct recrystallization and growth of huge petal-like nanocrystals, with Pt(111) or Pd(111) preferential orientation along the growth direction. The superior crystallinity of Type B-1 was also observed by selected area electron diffraction (SAED) patterns (Fig. S2) and x-ray diffraction (XRD) (Fig. S4). In contrast to Type B-1, which formed from Type A by increasing the reaction time, Type B-2 and Type B-3 were prepared by injecting a 33 wt% and 43 wt% mixed metal salt solution, respectively. The structure exhibited slender (Fig. 3(d)-2) and flatter (Fig. 3(e)-2) petal-like building blocks that spread radially, as shown in the HRTEM and HAADF images. Along with the change in architecture, there was a significant decrease in the proportion of Pt in the three Type B Pt-Pd FNAs (compare the EDX mappings in Fig. 3). The atomic ratio of Pd:Pt was approximately 85:15, 70:30, and 83:17 for Type B-1, Type B-2, and Type B-3, respectively. When the building blocks became bigger and formed petal-like morphologies with preferential orientations, the inhomogeneity in the atomic ratio of Pd:Pt along the growth direction remained. However, Pd became the only dominant component in both the core and shell, as evidenced by the EDX maps (Fig. 3) and line scans (Fig. S3). This is very different than the Pt-rich shell found in Type A. The composition gradient analyzed by X-ray photoelectron spectroscopy (XPS) also confirmed the observation above, by comparing the as-received and surface etched specimens (Fig. S5, Fig. S6 and numerical data listed in Table S1). Additionally, the asymmetry of the XPS peaks and the profile analyses of Pt and Pd elements are enlightening (Fig. S5). These results strongly suggest that the Pt components in both of Type A and Type B were oxidized, whereas the Pd one displayed relatively less oxidation.36-40 The



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noticeably different oxidation behaviors should be related to the distribution and reduction of Pt4+ and Pd2+ in the Pt-Pd FNAs. The very clear and sharp SEM or TEM images taken from the prepared Pt-Pd FNAs could evidently confirm that most F127 soft templates (carbon) were removed by repeated washing procedures as described in the experimental section. Although the XPS spectra (see Fig. S5) indicate the presence of slight amount of carbon, the corresponding intensity is close to the general level arising from the dust in the atmosphere. In addition, from the Raman analyses of the prepared PtPd FNAs (see Fig. S7(a)), two main carbon Raman peaks including D band and G band are almost invisible. In other words, the SERS signals of the carbon in the blank SERS platform can be ignored. The PEO and PPO segments in the F127 (PEO106-PPO70-PEO106) would spontaneously form micelles exhibiting a hydrophilic shell and hydrophobic core, respectively, as a result of the synthesizing procedures used for this study. The presence of intrinsic hydroxyl groups (-OH) at the end of the PEO segments, as well as the reactive aqueous solution, might thus lead to oxidation of the reduced metallic Pt and Pd close to the surface of Pt-Pd FNAs. In other words, a relatively high fraction of Pd located at the hydrophobic PPO core of the Pt-Pd FNAs (Fig. S3) could be effectively protected from oxidation. It was previously demonstrated that the PEOn-PPOm-PEOn tri-block copolymer could act not only as an efficient reductant for reducing metallic precursors but also as a stabilizer that prevents the aggregation of the reduced metal nanoparticles. At the same time, the polymer chains could also act as a growth director, by selectively capping specific surfaces of the grown metals to form a variety of dendritic morphologies.27-30 It should be noted that the concentration of the tri-block copolymers employed in those reports was, in general, lower than the critical micelle concentration (CMC) for averting the formation of poorly defined structures. When the concentration of the tri-block copolymers exceeds CMC, as described above, the gelled tri-block copolymers could spontaneously form special three dimentional periodic architectures which were frequently applied as soft templates. It is worth emphasizing that the present work aiming at synthesizing highly uniform Pt-Pd FNAs with an extremely high


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concentration of F127 solution (30−35 wt%) should be one of the most successful cases of demonstrating the unique role of soft templates of the tri-block copolymers. Similar investigations on the fabrication of Pt-Pd bimetallic nanostructures using a high concentration of F127 solution (> CMC) could merely find irregularly aggregated bimetal nanoparticles and nano-fractions.25 In the present work, the final size of the prepared Pt-Pd FNAs were evidently found that indeed varies with the atomic ratio of Pt/Pd precursors. The relevant SEM evidences (see Fig. S8) point out a lower Pt/Pd ratio would lead to a larger assembling size. Alternatively, another tri-block copolymer, P123, comprising (PEO)20-(PPO)70-(PEO)20, was also applied for the synthesis of the Type A Pt-Pd FNAs. The structure of P123 is very similar to F127, (PEO)106-(PPO)70-(PEO)106, but has two relatively short PEO ends. According to the phase diagram of P123 (see Fig. S9), a cubic packing zone of the P123 micelles can be found under specific P123 concentration and temperature, and was considered as a suitable condition for synthesizing Pt-Pd FNAs. The TEM images (see Fig. S9(c)) show the Type A Pt-Pd FNAs synthesized by P123, clearly indicating the possibility of successful synthesis of Pt-Pd FNAs using other similar tri-block copolymers. The formation mechanism of the highly uniform Pt-Pd FNAs (Type A) was investigated by exsitu small-angle X-ray scattering (SAXS). According to the phase diagram (see Fig. S9), F127 micelles would form FCC packing when the concentration of F127 solution reaches 35 wt% at 25oC as illustrated in Fig. 4(i). The scattering profiles of the ex-situ SAXS presented in Fig. 4(a) show wellresolved peaks. The relative peak position of 1:(4/3)1/2:(8/3)1/2:(11/3)1/2 is in good agreement with the characteristic values of the FCC packing.34 The diameter of the F127 micelle in the FCC packing was found to be 14.96 nm by using Bragg’s Law:

q = (4π/λ)sinθ = 2π/d



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where q is the scattering factor, λ the wavelength of the characteristic x-rays, θ the Bragg angle, and d the inter-planar spacing of the crystal. The gyration radius of F127 micelles (30 wt%) obtained from SAXS (see Fig. 4(a)) is slightly smaller than the reported value (20 wt%) owing to the difference in the concentration of F127 solution.34,41 When the metal salt solution was added into the F127 gel-like template, the metal ions would well disperse in the PEO shells of F127 micelles, which made F127 micelles swollen and lead to clearer frameworks of the FCC packing (see obvious peaks displayed in Fig. 4(a), 2 days to 6 days). The reduction preferentially occurred along the space between neighboring micelles. More precisely, the space coincided with the region constructed by connecting PEO shells of all the FCCpacked F127 micelles (gel state). The intensity of ex-situ SAXS profiles is considerably improved along with reaction time (2 to 6 days) and a growing amount of reduced metal p-NPs in micelle reaction sites. The radius increased by swollen F127 micelles goes with the low q shift of the SAXS curve for the mixture of F127 micelles and Pt-Pd p-NPs. Supported by HRTEM images (see Fig. 5) and previous reports 42-43, interference broad shoulder at q around 1.3 nm-1 is due to intraparticle boundary located between micellar core and shell.(see Fig. S10). The sharp boundary between PEO-core and PEO-shell enhanced by Pt-Pd p-NPs reveals that Pt-Pd p-NPs tend to be started the reduction near PPO core area. It is notable that, for the first time, the Pt NPs were reduced directly by a tri-block copolymer without the assistance of ascorbic acid or ultrasonic energy. Reacted in extraordinary high concentrated F127 solution, prepared Pt NPs were clearly observed in gathered F127 micelles shown in HRTEM images (Fig. 5). Since Pd would be reduced easier, and consequently faster, than Pt in the same medium and condition, it is thus reasonable to consider that the initially formed Pd NPs served as auto-catalysts, to expedite the growth of the Pt NPs. In other words, all Pt-Pd FNAs should start from initial Pd nuclei, followed by a Pd-rich core. Metallic Pt should almost simultaneously appear with Pd for both Type A and Type B (Fig. S3). More PEO chains were involved in the reaction with the Pd auto-catalyst, which caused the aligned petal-like nanocrystals, as observed in Type B. The slope of shoulder presented in


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SAXS curves is -4 which is in agreement with the HRTEM images that the Type A Pt-Pd FNAs are the spherically assembles of spherical p-NPs. It is interesting that when the reaction time was extended to over few tens of days, the formed FCC-packed F127 soft-template would completely collapse probably due to the overgrowth of the PtPd FNAs, as evidenced from the only presence of two very broad shoulders around 0.53 nm-1 (F127 micelles) and 1.35 nm-1 (p-NPs) (see Fig. 4(b), 45 days to 49 days). The slope of -4 obtained from the low-q region indicates the synthesized spherical Pt-Pd FNAs were maintained. According to the TEM images and ex-situ SAXS analyses, the schematic diagram describing the proposed mechanism of forming such unique Pt-Pd FNAs is shown in Fig. 4. The synthesis began from a 35wt% F127 solution containing a specific amount of Pt and Pd precursors in an ice bath. When the solution was warmed from 0 oC to 25 oC, the suspended F127 micelles would spontaneously self-assemble into FCC packing (see Fig. 4-(i)) as can be predicted by the C-T phase diagram of F12735 (see Fig. S9). Among the FCC packed F127 micelles, although the mixture of F127 and metallic precursors was stirred for over ten minutes, the distribution of Pt4+ and Pd2+ ions should not be exactly consistent everywhere due to the extremely high viscosity of the gelled F127 micelles at 0 oC and also the packed F127 micelles at 25 oC, as well as the nanoscale phase separation caused by the coexistence of the hydrophilic and hydrophobic regions. The reduction should take place first in the hydrophilic regions (PEO segments) containing a higher concentration of Pd2+. At the same time, the formed Pd p-NPs might act as catalysts to trigger the reduction of Pt4+. Based on previous reports and also our experience, with the absence of catalysts, i.e. Pd p-NPs in the present case, the Pt4+ ions cannot be reduced in F127 solution at room temperature in general. The reduced Pd and Pt p-NPs would then start to self-assemble into Pt-Pd clusters or pre-FNAs (see Fig. 4-(ii)). The Pt-Pd clusters would continuously grow up by gathering more and more Pd and Pt p-NPs (see Fig. 4-(iii)). The relatively hydrophobic feature of the formed Pt-Pd clusters in the hydrophilic PEO bath might also lead to a spontaneous combination of the neighbor clusters. The growth and assembling progress should occur in or along the PEO regions and would be reasonably restricted due to the FCC packed



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F127 micelles. As proved by the ex-situ SEM images (see Fig. 7), it is clear that the size of Type A Pt-Pd FNAs became more uniform along with the reaction time. In other words, the final size of the Pt-Pd FNAs would be moderated by the space confinement effect induced by the FCC packed F127 micelles. The extremely uniform morphology and size of all types of the Pt-Pd FNAs are thus understandable (see Fig. 3(a) and Fig. 7). Accompanying with the final growth stage, it was found that the jelly-like F127 soft-template gradually melted into liquid, indicating the collapsing of the F127 soft-template. The template collapsing phenomenon is understandable since the entangled PEO networks within the FCC-packed F127 micelles might gradually break with the growth of the Pt-Pd FANs. In addition, the growing hydrophobic Pt-Pd FANs might also lead to relocation or phase separation of the hydrophilic F127 micelles (see Fig. 4-(iii) and Fig. 6(left)) and the consequent collapsing of the F127 soft-template (see Fig. 6 (right)). As described above, well-separated primary metallic nanocrystals in the Pt-Pd FNAs could potentially serve as densely packed hot spots for enhancing SERS signals. The unique featured morphologies of both Type A and Type B Pt-Pd FNAs also lead to significant changes in surface plasmon resonance (SPR) which has become an important label-free optical sensing technology in numerous areas as SERS. In contrast to spherical Pt or Pd nanoparticles typically revealing narrow SPR peaks at the invisible region, the present Pt-Pd FNAs exhibit extremely broadening and significantly red-shifted SPR peaks (see Fig. 8), which might be beneficial for allowing the Pt-Pd FNAs to be optically activated using common excitation laser source. To investigate the SERS performance of the prepared Pt-Pd FNAs, malachite green (MG) was selected as a probe molecule. A 20 μL droplet of the MG solution was mixed with 40 μL Pt-Pd FNAs suspension aqueous solution under ultrasonic for 30 seconds. The mixture was dropped into a glass slot (see the inset of Fig. 9(d)) and was then enclosed with a coverslip for spot-to-spot micro-Raman spectrum measurement which was carried out in a backscattering geometry using an YVO4 double laser (Verdi, coherent. Inc.) as the excitation source (532 nm) coupled to an HORIBA JOBIN YVON spectrometer equipped with 100X objective and the liquid nitrogen CCD detection. To approach consistent analyses, the weight


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concentration of the re-suspended three-dimensional hierarchical Pt-Pd nanoparticles and also the analytical procedures as described above were precisely controlled. Figures 9(a)-(c) present the SERS spectra of the three type Pt-Pd FNAs with MG solutions from 100 ppm to 1 ppm. It was found that all the characteristic peaks of MG could be clearly observed even for 1 ppm (see the case of Type B-3 PtPd FNAs). In addition, the S/N ratios shown in Fig. 9d are the average values of at least four independent SERS spectra. We confirmed that all the SERS spectra, as well as the corresponding S/N ratios displayed in the present work, are reproducible even using the nanomaterials from different synthesis batches. Compared to MG powders (Fig. S7), the characteristic peaks are extremely weak for 10 ppm aqueous MG solution. The SERS peak at 1622 cm-1 was obviously elevated while introducing the PtPd FNAs into the MG aqueous solution. It has been previously demonstrated that Pt, Pd or their alloys colloidal solution whose SPR cannot be effectively excited by visible light are not the best options for SERS applications typically using visible lasers as the Raman excitation source.44-45 Colloidal Au or Ag nanocrystals exhibiting strong SPR peaks in the visible light region thus become indispensable components for most SERS sensors for amplifying Raman signals arising from target molecules.46-49 In other words, SERS sensing performed with nanostructures under solution phase merely comprising Pt, Pd, or their alloys remains a challenge. In contrast to the strategy of using colloidal Au or Ag nanocrystals as SERS enhancers, amazingly, the present colloidal Pt-Pd FNAs evidence that with appropriate architectural designs, the Pt-Pd binary nanostructures could reveal wide SPR absorption bands covering the visible light region (see Fig. 8) and thus could act as promising SERS sensors without the addition of Au or Ag components. Figure 9(d) illustrates the S/N ratio with standard deviation for the main SERS peak at 1622 cm1

for different MG concentrations with the three featured Pt-Pd FNAs: Type A, Type B-2, and Type

B-3. Obviously, Type A and Type B-3 presented a better signal enhancement than Type B-2 over the whole measurement range. The limit of detection (LOD) even approaches below 1 ppm for Type B-3. It is notable that nanocrystals with sharp tips, or dendrites, could act as great media for enhancing



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SERS performance with reproducibility.50 The signal enhancement was related to the edge structure of the building blocks in these three types of Pt-Pd FNAs. The relatively limited improvement found for Type B-2 was probably due to the densely assembled slender petal-like building blocks, which led to a very smooth top surface and tiny gaps (Fig. S11) for inhibiting the inward diffusion of the targeting molecules. The enhancement factor (EF) can be calculated to evaluate SERS sensitivity using the equation below51-56: 𝑬𝑭 =

( )/ ( ) = 𝑰𝑺𝑬𝑹𝑺

𝑰𝑹𝒆𝒇

𝑰𝑺𝑬𝑹𝑺𝑵𝑹𝒆𝒇

𝑵𝑺𝑬𝑹𝑺

𝑵𝑹𝒆𝒇

𝑰𝑹𝒆𝒇 𝑵𝑺𝑬𝑹𝑺

where ISERS and IRef are the Raman intensity of the specimen and reference, respectively. NSERS and NRef are the numbers of target molecules of the specimen and reference illuminated by the laser, respectively. In this study, ISERS is the Raman intensity (1622 cm-1; the main characteristic Raman peak of malachite green (MG)) obtained from the mixture of the malachite green solution and the prepared different PtPd FNAs, and IRef is the intensity of the same Raman peak (1622 cm-1) recorded from 100 ppm MG solution. The EFs obtained from these prepared Pt-Pd 3D-FNAs are around 10 which are obviously lower than those found from the most famous SERS materials, i.e. colloidal Au or Ag nanoparticles. In comparison with the significantly high EFs from the known most effective SERS materials, e.g. colloidal Au or Ag nanoparticles, the EFs of around 10 obtained from these prepared Pt-Pd 3D-FNAs which were applied as liquid-state SERS sensors in the present work might originate from the relatively large size, or more precisely, the smaller surface-to-volume ratio, of each single Pt-Pd 3D-FNAs. It is thus reasonably expected that the EFs of the Pt-Pd 3D-FNAs could be further enhanced as they are applied as solid-state dry SERS sensors. In addition, the present Pt-Pd 3D-FNAs might act as SERScatalysis dual active platforms for SERS study of difficult to study catalyzed reactions.

Conclusions


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In this work, a series of highly-uniform Pt-Pd binary quasi-orderly assembled nanostructures, namely Pt-Pd FNAs, have been successfully synthesized as a potential candidate for alternating conventional SERS sensors including suspended or patterned Au or Ag nanocrystals. The structural, compositional and optical characteristics of the prepared complicated Pt-Pd FNAs have been carefully investigated, and at least two categories of Pt-Pd FNAs, Type A and Type B (3 types), respectively comprising spherical and petal-like building nanocrystals, could be accordingly classified and reproducibly fabricated, evidently confirming the original concept and synthesis strategy of using dual metallic precursors as well as the tri-block copolymer, F127, as the reductants, and more importantly, the micro-reactors. The significantly broadening SPR spectra almost completely covering visible light region reasonably originate from the extremely porous assembling of the uniform distinct Pt-Pd nanocrystals. The present colloidal Pt-Pd binary materials system reveals identifiable performance in sensing aqueous MG with a remarkably low detection limit of less than 1 ppm. The reproducible SERS performance of colloidal Pt-Pd FNAs was proved, alternatively, the quantitative SERS sensing of dropcoasting Pt-Pd FNAs chip will be further investigated. The present study not only provided new insight into the innovative concept regarding the SERS enhancement with quasi-ordered Pt-Pd FNAs but more importantly, experimentally confirmed its feasibility. The concept, as well as the relevant synthesis strategies, would practically reduce the dimension of SERS sensing devices to single FNA and is believed to be a milestone for promoting the development of next-generation SERS sensors for a variety of valuable applications.

Experimental section The preparation of the Pt-Pd FNAs (Type A) was as follows: PdCl2 was dissolved in deionized water with HCl for producing H2PdCl4 solution. A small vial that contained 6 g of 35 wt% Pluronic F127(aq) was placed in an ice bath and mixed with 0.6 g of 20 mM H2PtCl6 and 0.4 g of 20 mM H2PdCl4



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under stirring. The final concentration of F127 in the mixture solution was 30 wt%, and the well-mixed solution transformed into the gel state after it was placed in water at 25 oC. The mixture color turned from orange to dark brown in few minutes and indicated that Pd and Pt nanoparticles were produced hierarchically in the template made by F127 micelles. Type A Pt-Pd FNAs were prepared when the gel mixture transitioned to a dark liquid. The reaction time takes about seven days. It was collected by centrifugation, purified by deionized water then dispersed with 2 mL deionized water in a small vial for further characterization. The composition of Type A Pt-Pd FNAs was Pt40Pd60, as characterized by EDX. Petal-like Type B Pt-Pd FNAs could be transformed from Type A Pt-Pd FNAs directly, or by following the same synthesis strategy, but changing the reactant ratio of the metal salts solution and F127(aq) solution. After keeping the Type A nano-assemblies dispersed in the dark mother liquid over a period of time (about two months), the dark solution would turn to light yellow with a black precipitate. After centrifugation, purification and characterization, the composition of the black precipitate was determined to be Pt15Pd85, and called a Type B-1 Pt-Pd FNAs. Another preparation of the Type B Pt-Pd FNAs was to add different wt% of Pt/Pd precursors to F127 solution. Two examples, Type B-2, and Type B-3 Pt-Pd FNAs, were synthesized as follows respectively: 35 wt% and 30 wt% F127 solution were prepared in small vials separately and placed in the ice bath. This was followed by 20 mM H2PtCl6 and H2PdCl4 being dropped into the vials, to make the final concentration of F127 solution for both to equal 20 wt%. The two well-mixed solutions transitioned to the gel phase after placing in water at ambient temperature. The color turned from yellow to black-brown in ten minutes. The reaction was done when the gel mixture changed to a brown-orange liquid with a black precipitate. The composition of the black precipitate was close to Pt15Pd85, as prepared from the Type A Pt-Pd FNAs. All the products were obtained by centrifugation at 15000 rpm for 30 min followed by consecutive washing/centrifugation cycles five times with water. The collected products were redispersed in water with sonication for further characterization.


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The preparation of the Pd FNAs was followed the same procedure as Type A but merely utilized H2PdCl4(aq) solution as metal precursor. The concentration of F127(aq) solution was started from 35 wt% and ended at 30 wt% after adding H2PdCl4 solution. The well-mixed solutions transitioned to the gel phase after placing in water at ambient temperature. The color turned from orange to black-brown in ten minutes. The reaction was done after seven days. It was collected by centrifugation, purified by deionized water then dispersed with 2 mL deionized water in a small vial for further characterization. The TEM image and extinction spectra of the Pd FNAs were shown in Fig. S12. The morphologies of the Type A and Type Bs nanostructures were characterized by highresolution transmission electron microscopy (HRTEM) using a JEOL JEM-2100F operated at 200kV equipped with an energy dispersive spectrometer analyses (EDS). Wide-angle powder X-ray diffraction patterns were obtained with Bede D1 with monochromated Cu Kα radiation (45 kV, 35 mA). In-situ SAXS measurements were performed using a Bruker Nanostar SAXS instrument. The Xray source, a 1.5 kW X-ray generator (Kristalloflex 760) equipped with a Cu tube, was operated at 35 mA and 40 kV. All scattering data were corrected by the empty beam scattering, the sensitivity of each pixel of the area detector, and thermal diffuse scattering (TDS). The thermal diffuse scattering was considered as a positive deviation from the Porod law. The SAXS experiments were performed at the BL23A beamline of National Synchrotron Radiation Research Center (NSRRC), Taiwan.57 UV-vis absorption spectra were recorded using a Hitachi U-3900 UV-vis spectrometer. Micro-Raman measurements were carried out in a backscattering geometry using YVO4 double laser (Verdi, coherent. Inc.) excitation source emitting at 532 nm coupled to a HORIBA JOBIN YVON spectrometer equipped with 100X objective and the liquid nitrogen CCD detection.

Conflicts of interest There are no conflicts to declare.

Acknowledgments



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Financial support from the Ministry of Science and the Technology of the Republic of China under grant 105-2622-E-009-028-CC2 is gratefully acknowledged.

Supporting Information Available: The detail of XPS analysis, enhancement calculation and HRTEM images of Pt-Pd FNAs and related experiment were described in supplementary.

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Figure 1. Schematic diagrams of the fundamental concept of p-NPs assembled Pt-Pd FNAs served as three-dimensionally integrated SERS sensors for magnifying Raman signals.

Figure 2. Schematic diagrams of the key concept for synthesizing two featured Pt-Pd FNAs, Type A and Type B, respectively comprising self-assembled spherical and petal-like nanocrystals. The scale bar simply presents the composition distribution of the Pd and Pt components.



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Figure 3. (a-1) TEM images taken from the synthesized Pt-Pd FNAs (Type A) and (a-2) larger magnification of (a-1). TEM, HAADF, and EDS (Pt: green, Pd: red, and C: blue) analyses aiming at single Pt-Pd FNA assembly of (b-1 to b-3) Type A, (c-1 to c-3) Type B-1, (d-1 to d-3) Type B-2, and (e-1 to e-3) Type B-3, respectively.


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Figure 4. (a) Ex-situ time-resolved SAXS patterns of 30 wt% Pluronics F127 without and with the reduced Pt-Pd nanoparticles. (b) Ex-situ time-resolved SAXS patterns recorded from the F127 collapsing stage of the synthesis. Associated with the SAXS patterns, the schemes show (i) the FCC-packed high concentration (30wt%) F127 soft-template and the proposed growth mechanisms of Pt-Pd FNAs: (ii) reduced Pt-Pd p-NPs with F127 micelles, (iii) gradual migration of F127 micelles with as-synthesized Pt-Pd FNAs and (iv) synthesized Pt-Pd FNAs in collapsed F127 soft-template.



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Figure 5. TEM images of Pt nanoparticles formed in 30 wt% Pluronic F127 soft-template after being reduced for (a) three days, and (c) two months. The larger magnification of single micelle selected from (a) was presented in (b).

Figure 6.

The proposed collapsing mechanism of the FCC-packed F127 soft-template.


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Figure 7. SEM images of Type A Pt-Pd FNAs synthesized with different reaction time: (a) 12 hours, (b) 2.5 days and (c) collapsed-gel state.

Figure 8. The extinction spectra of the synthesized Pt-Pd FNAs with three featured p-NPs morphologies: (a) Type A, spherical p-NPs; (b) Type B-2, flatter petal-like p-NPs; (c) Type B-3, slender petal-like p-NPs.



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Figure 9. Raman spectra recorded from water-suspended (a) Type A Pt-Pd FNAs, (b) Type B-2 Pt-Pd FNAs and (c) Type B-3 Pt-Pd FNAs well-mixed with varied concentration of the malachite green solution. (d) The S/N ratio calculated from the SERS peak at 1622 cm-1 for different MG concentrations with the prepared three featured Pt-Pd FNAs. The MG concentration presented here was recalculated to present the real amount of in measurement. The inset is the photograph of the SERS sensor.


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