A novel pH-responsive Ag@polyacryloyl hydrazide (Ag@PAH) nanoparticle for the first time as a surface-enhanced Raman scattering (SERS) substrate was prepared without reducing agent and end-capping reagent. Ag@PAH nanoparticles exhibited an excellent tunable detecting performance in the range from pH = 4 to pH = 9. This is explained that the swelling-shrinking behavior of responsive PAH can control the distance between Ag NPs and the target molecules under external pH stimuli, resulting in the tunable LSPR and further controlled SERS. Furthermore, Ag@PAH nanoparticles possessed an ultra-sensitive detecting ability and the detection limit of Rhodamine 6G −12 reduced to 10 M. These advantages qualified Ag@PAH NP as a promising smart SERS substrate in the field of trace analysis and sensors. Keywords: pH-responsive, Ag@PAH NPs, SERS, Ultra-sensitive, Tunable Background means the characteristic fingerprint of target molecules Surface-enhanced Raman scattering (SERS) is a powerful can be acquired even at low concentrations [8–10]. spectroscopic tool to identify molecule structure by vi- To date, considerable efforts have been devoted to brational information of target molecules . Due to its improve the sensitivity of SERS to develop the technique convenience and ultra-sensitive analysis, SERS has been of SERS analysis. The successful strategies for ultra- recognized as an ideal approach to detecting biological sensitive SERS have been realized by metal nanoparticle molecules, including DNA, RNA and cancer cells . It is substrates with different shapes and dimensions . generally agreed that SERS technique can be illustrated However, to our knowledge, there are no corresponding with the enhanced electromagnetic (EM) . Among the reports about the controllable SERS detection [12–15]. influences of EM, the localized surface plasmon resonance Therefore, developing tunable SERS will become one of (LSPR) plays a key and dominant role . When target the greatest challenges associated with high sensitivity molecules reside in the gaps between neighboring metal SERS and biosensors. Polyacryloyl hydrazide (PAH) is a nanoparticles (so-called “hot spots”), under the irradiation pH-responsive polymer, which has been applied to vari- of incident light, the metal nanoparticle generates LSPR ous biomedical fields . Owing to abundant hydrazide and its surface electromagnetic field is increased, resulting functional groups on PAH, PAH can serve as not only in the enhanced signal of SERS [5–7]. The enormous the end-capping reagent but also the reducing agent of enhancement ensures the high sensitivity of SERS, which the metal ion precursors to easily prepare Ag nanoparti- cles (NPs) . The swelling-shrinking behavior of responsive PAH can control the distance between Ag NPs and the target molecules under external pH stimuli, * Correspondence: firstname.lastname@example.org resulting in the tunable LSPR and further controlled College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 201620 Shanghai, People’s Republic of China SERS. Key Laboratory of Textile Science & Technology, Ministry of Education, Donghua University, Shanghai, People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Yuan et al. Nanoscale Research Letters (2017) 12:490 Page 2 of 5 In this work, by combining pH-responsive PAH polymer was 102.6 nm at pH = 9 and 13.3 nm at pH = 4. The and Ag NPs, we successfully prepared Ag@PAH NPs reason should be attributed to the swell and shrink of without other reagents. Rhodamine 6G (R6G) as the target the PAH. The swell and shrink of the PAH attributed to a molecule, Ag@PAH NPs were used to SERS detection for synergistic effect of the following factors, protonation- the first time. Due to the responsive of PAH polymer on deprotonation change, charge repulsion, and the hydrogen- the surface of Ag NPs, a controllable SERS effect of the bond forming capacity of PAH polymer. In addition, the R6G/Ag@PAH NPs can be realized by adjusting pH value. Ag@PAH NPs showed similar absorption peak (at about Furthermore, Ag@PAH NPs exhibit a high sensitivity and 423 nm) in UV-vis spectra and only the absorption inten- reproducibility, which allow them to be explored for sity decreased in the pH range from 4 to 9 in Fig. 2c. This biological hazards or chemical reagent analysis in field indicated the increasing thickness of polymer shell layer applications. would hinder the spread of the localized surface plas- mon resonance without changing the optical property Methods of Ag NPs. The illustration of the prepared process of Ag@PAH NPs The SERS performance of Ag@PAH NPs was evaluated was shown in Fig. 1. Briefly, 250 μLAgNO aqueous solu- with R6G as the model target analyte. In order to under- tion (0.2 mol/L) was added to 25 mL PAH (ESI† for details) stand the origin of the Ag@PAH NPs enhancing R6G aqueous solution (2% w/v). The mixture was stirred under Raman signals, compared experiments were performed to amild conditionfor 30 minat30°C. Theresulting reddish distinguish the influence of the PAH polymer layer. We brown solution was purified by dialysis against deionized compared the Raman signals of the pure R6G solution, water for 24 h and collected by centrifugation and dis- pure PAH solution, individual Ag NPs and Ag@PAH NPs, persed in deionized water. Then, the different pH values of all of which had the same concentration in Fig. 3a. It is well −6 Ag@PAH NP solutions were adjusted by 0.1 mol/L HCl known that the signal of the pure R6G solution (10 M) is solution or 0.1 mol/L NaOH solution. quite weak. After adding Ag NPs or Ag@PAH NPs as sub- strates, the main characteristic peaks at 1311, 1363, 1509 −1 Results and Discussion and 1651 cm , which perfectly matched the Ramam The PAH polymer possessed hydrazide groups in each spectra of R6G were obviously enhanced. This demon- repeating unit, which served as an effective reducing strates that a remarkable SERS signals from R6G molecules agent for preparation of metal NPs . Ag electro- present on the surface of Ag NPs and Ag@PAH NPs. In philic substitution, the nitrogen at the end of hydrazide contrast, in the absence of Ag NPs, negligible SERS signals groups, formed -CO-NH-NH- and Ag NPs, in the prep- were observed from individual PAH polymer, suggesting aration process of Ag@PAH NPs. By high-resolution that the presence of PAH polymer had no effect on the transmission electron microscopy, we found that the Ag SERS effect for R6G molecules. NPs were fully encapsulated by PAH polymer with the The SERS enhancement of metal cell/polymer shell complete core-shell structure. We further estimated that was very sensitive to the polymer shell thickness, which the average size of Ag NPs was about 90 nm in Fig. 2a. has been proved by both theoretical and experimental The hydrodynamic diameter of the Ag@PAH NPs was studies. We investigated the effect between different pH 192.6 nm at pH = 9 and decreased to 103.3 nm when values and SERS-enhanced signals as expressed in Fig. the pH value was 4 in Fig. 2b. Moreover, we further cal- 4a. Compared with the original signal of R6G, the SERS culated the thickness of PAH shell by subtraction of the signals were amplified in the presence of Ag@PAH NPs Ag NP diameter from the total of Ag@PAH NPs which at different pH conditions. Furthermore, the relative Fig. 1 Schematic illustration of the prepared process of Ag@PAH NPs Yuan et al. Nanoscale Research Letters (2017) 12:490 Page 3 of 5 Fig. 2 a HRTEM images and particle size distribution of Ag@PAH NPs. b pH dependence of the hydrodynamic diameter of Ag@PAH NPs. c pH dependence of the UV-vis absorption spectra of Ag@PAH NPs SERS intensity of the spectra dropped as the pH value concentration. Comparing the signals of these curves, the increases. This is explained that SERS effect of Ag@PAH SERS intensities were decreased by diluting the concentra- NPs was sensitive to the shell thickness of PAH. PAH tions of the target molecule in Fig. 4c. The characteristic shell layer shrank at low pH value, resulting in more bands of R6G are identified clearly even at a concentration −12 intensity of electromagnetic field than that at high pH as low as 10 M, demonstrating Ag@PAH NPs possess a value in the same concentration of Ag@PAH NPs, as high detected sensitivity for R6G. Furthermore, a linear show in Fig. 3b. Therefore, the Ag@PAH NPs at low pH dependence is found between the logarithmic concentra- induced extremely enhanced Raman signals, which tions of R6G and the intensities of the fingerprint peak −1 ensured tunable of the Ag@PAH NPs as SERS substrates. (1509 cm ) in Fig. 4d. When in the concentration range −7 −12 This phenomenon was quantified by calculating the Raman of R6G ranged from 10 to 10 M, the linear regression −1 enhancement factors (EFs) of the 1509 cm peak for equation was y = 5.9838 + 0.3228 log(x), and the correl- Ag@PAH NPs (Eq. S1, ESI†). The EFs of Ag@PAH NPs at ation coefficient was 0.9971 (n = 6). Obviously, in the low different pH values were estimated to be 0.8 × 10 , concentration region, SERS intensity decreased with the 6 6 6 6 6 1.1 × 10 ,1.5 × 10 ,2.2 × 10 ,3.3 × 10 and 4.3 × 10 , test concentration decreases. These results confirmed that respectively, in Fig. 4b (ESI† for details). The EFs of the Ag@PAH NPs will become a promising candidate in a Ag@PAH NPs at different pH values were all high, up to smart ultra-trace detection of biological hazards or chem- 10 which revealed that the Ag@PAH NP could be used as ical reagents. an effective and intelligent SERS substrate in the trace detection. Conclusions In addition, Ag@PAH NPs at low pH value induced In summary, we utilized pH-responsive Ag@PAH NPs extremely enhanced Raman signals, which ensured as desired substrates for SERS applications for the first ultra-sensitivity of the Ag@PAH NPs as SERS substrates. time. The introduction of pH-responsive PAH polymer Therefore, a series of SERS spectra of R6G at different as a shell layer can endow Ag NPs a controllable local- −7 −12 concentrations (10 –10 M) were further measured at ized surface plasmon resonance by adjusting the shell pH = 4 with adding Ag@PAH NPs at the same thickness under pH stimuli, resulting in tunable SERS Fig. 3 a Schematic illustration of the fabrication SERS process of R6G on Ag@PAH NPs substrates. b Schematic illustration for the tunable SERS mechanism of R6G with Ag@PAH NPs at different pH values Yuan et al. Nanoscale Research Letters (2017) 12:490 Page 4 of 5 −1 Fig. 4 a SERS spectra of R6G adsorbed with different pH values. b EFs of R6G on Ag@PAH NPs as a function of pH values at 1509 cm . c SERS −1 spectra of R6G with different concentrations adsorbed on Ag@PAH NPs. d Relationship of peak intensities at 1509 cm and concentrations of R6G (The inset is the linear relationship between the logarithmic intensities and concentrations of R6G.) effects. The results demonstrated that Ag@PAH NPs contributed to the data interpretation, manuscript writing and supervised the research. All authors read and approved the final version of the manuscript. possessed excellent controllable pH-responsive and ultra- sensitive SERS performance which the detection limit of −12 Competing Interests R6G reduced to 10 M. Ag@PAH NPs are promising for The authors declare that they have no competing interests. the smart SERS application in the ultra-trace detection of biological hazards or chemical reagents. Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in Associated Content published maps and institutional affiliations. Supporting information. Materials, intstrumentation, preparation of PAH and EF caculation method. Figure S1. Received: 19 June 2017 Accepted: 29 July 2017 H NMR spectrum of PMA in CDCl and PAH in D O 3 2 (Additional file 1). References 1. Kurouski D, Duyne RPV, Chem A (2015) In situ detection and identification Additional file of hair dyes using surface-enhanced Raman spectroscopy (SERS). Anal Chem 87(5):2901–2906 Additional file 1: Supplementary material. (DOCX 265 kb) 2. Wang Y, Salehi M, Schütz M, Schlücker S (2014) Femtogram detection of cytokines in a direct dot-blot assay using SERS microspectroscopy and hydrophilically stabilized Au-Ag nanoshells. Chem Commun 50(21):2711–2714 Abbreviations 3. 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Nanoscale Research Letters – Springer Journals
Published: Aug 14, 2017
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