Article pubs.acs.org/ac
A Poly Adenine-Mediated Assembly Strategy for Designing SurfaceEnhanced Resonance Raman Scattering Substrates in Controllable Manners Ying Zhu,†,§ Xiangxu Jiang,†,§ Houyu Wang,†,§ Siyi Wang,† Hui Wang,† Bin Sun,† Yuanyuan Su,† and Yao He*,†,‡ †
Institute of Functional Nano & Soft Materials (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, China ‡ Department of Chemistry, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: In this article, we introduce a Poly adenine (Poly A)-assisted fabrication method for rationally designing surface-enhanced resonance Raman scattering (SERRS) substrates in controllable and reliable manners, enabling construction of core−satellite SERRS assemblies in both aqueous and solid phase (e.g., symmetric core (Au)-satellite (Au) nanoassemblies (Au−Au NPs), and asymmetric Ag−Au NPs-decorated silicon wafers (Ag−Au NPs@Si)). Of particular significance, assembly density is able to be controlled by varying the length of the Poly A block (e.g., 10, 30, and 50 consecutive adenines at the 5′ end of DNA sequence, Poly A10/A30/A50), producing the asymmetric core−satellite nanoassemblies with adjustable surface density of Au NPs assembly on core NPs surface. Based on quantitative interrogation of the relationship between SERRS performance and assemble density, the Ag−Au NPs@Si featuring the strongest SERRS enhancement factor (EF ≈ 107) and excellent reproducibility can be achieved under optimal conditions. We further employ the resultant Ag−Au NPs@Si as a high-performance SERRS sensing platform for the selective and sensitive detection of mercury ions (Hg2+) in a real system, with a low detection limit of 100 fM, which is ∼5 orders of magnitude lower than the United States Environmental Protection Agency (USEPA)-defined limit (10 nM) in drinkable water. These results suggest the Poly A-mediated assembly method as new and powerful tools for designing high-performance SERRS substrates with controllable structures, facilitating improvement of sensitivity, reliability, and reproducibility of SERRS signals.
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assembled core (Au)-satellite (Au) nanostructures (Au−Au NPs) based on strong covalent Au−S bonds, leading to a strong plasmonic coupling and high EF values (EF ≈ 105).6 Quite recently, Tian et al. presented three-dimensional (3D) SERS hotspot matrix generated by evaporating a droplet containing Ag−Ag NP hybrids on a silicon wafer, featuring minimal polydispersity of particle size and maximal uniformity of interparticle distance, which produced SERS enhancement of ∼2 orders of magnitude larger than that of dried SERS substrates.7 Meanwhile, Joo’s group fabricated Au−Au NPs with subnanometer gaps by encapsulating and closely packing into silica nanotube peapod, which exhibited superior SERS activity with an EF value of ∼3.1 × 107.8 These exciting achievements have demonstrated that assembled Au/Ag NPs with higher density could produce more coupled hot spots,
old and silver nanoparticles (NPs) are widely recognized as well-established surface-enhanced Raman scattering (SERS) substrates. Many studies have revealed that multiple hot spots of Au/Ag-based SERS substrates could largely contribute to the enhancement factor (EF).1−3 In addition, efficiently coupled hot spots are considered as another significant contributor to the EF, which could be readily obtained via controlling the interparticle distance. Accordingly, several strategies have been developed to produce Au/Ag assembly with strong plasmonic coupling of enormous hot spots.4−10 For instance, in 2008, Nie et al. designed highperformance SERS beacons based on long-range plasmonic coupling of Au−Au NP hybrids originated from hybridization of thiolated DNA-linked Au nanocrystals, in which 40−50-fold higher SERS signals were obtained, compared with single DNA-Au NP.4 In the following year, Chen et al. obtained higher SERS signals (EF ≈ 105−107) from dimers and trimers of Au@Ag core−shell NPs than that (EF ≈ 103) produced by monomers of Au@Ag NPs.5 In 2013, Schlücker and co-workers © XXXX American Chemical Society
Received: February 17, 2015 Accepted: May 31, 2015
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DOI: 10.1021/acs.analchem.5b00676 Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
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Article
EXPERIMENTAL SECTION Preparation of Au NPs. Au NPs with different sizes were achieved via citrate reduction.16,17 In brief, trisodium citrate (1%) was added to a boiling, rapidly stirred solution of 0.01% HAuCl4. The solution was kept boiling and stirred for 20 min, and then cooled to room temperature. The as-prepared Au NPs were stored at 4 °C. Preparation of the Ag NPs-Decorated Si Wafer (Ag NPs@Si). The Ag NPs@Si was synthesized via in situ growth of Ag NPs on silicon wafers based on a HF-etching method.18,19 In detail, the Si wafer was cleaned with acetone by ultrasonic treatment for 10 min, and washed with Milli-Q water for three times. The silicon wafer was then cleaned with H2SO4−H2O2 (3:1, v/v) for 30 min and rinsed with Milli-Q water three times, to remove organics. The cleaned silicon wafer was further immersed in HF (5%) for 30 min to achieve the silicon wafer covered by Si−H bonds. This modified silicon wafer was then placed into AgNO3 with slowly stirring for 90 s, achieving the Ag NPs@Si. The resultant Ag NPs@Si was dried with a gentle flow of nitrogen. Fabrication of Au−Au NPs Using a Poly A-Mediated Strategy. The as-prepared Au NPs of ∼100 nm as core nanoparticles were incubated with 50 nM recognizing DNA (P1), whose opposite ends were dually linked with Poly A (e.g., Poly A10, Poly A30, or Poly A50) and Raman reporter of Cy5 (see Table S1 in the Supporting Information, Poly A10/A30/ A50-P1-Cy5) for 16 h. Meanwhile, the Au NPs of ∼13 nm as satellite nanoparticles were incubated with Poly A30-P2 (the complementary DNA of P1; see Table S1 in the Supporting Information) in a molar ratio (DNA/Au NPs) of 200 for 16 h. The mixtures of DNA and Au NPs then were incubated in the buffer (pH 7.4) containing 10 mM sodium phosphate and 0.1 M NaCl for 40 h to further facilitate the formation of DNA-Au NPs. The excess DNA then was removed by centrifugation (12 000 rpm, 20 min, 4 °C). Afterward, such two resultant DNA-Au NPs conjugates were mixed in the hybridization buffer (0.3 M PBS, pH 7.0) for 24 h to form the asymmetric core−satellite nanoassemblies of Au−Au NPs through the hybridization between P1 and P2. Finally, the nonhybridized Au NPs were removed via rinsing with PBS buffer, followed by centrifugation (8000 rpm, 10 min, 4 °C). The statistical analysis of the number of satellite Au NPs around one core Au NP was performed by counting 10 random core−satellite NPs in each corresponding SEM image (relative standard deviations (RSDs) of
