Amino Acid-Assisted Synthesis of Hierarchical Silver Microspheres for

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Amino Acid-Assisted Synthesis of Hierarchical Silver Microspheres for Single Particle Surface-Enhanced Raman Spectroscopy Leilei Kang,† Ping Xu,*,†,‡ Dengtai Chen,† Bin Zhang,† Yunchen Du,† Xijiang Han,*,† Qing Li,‡ and Hsing-Lin Wang‡ †

Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

‡

S Supporting Information *

ABSTRACT: We demonstrate the use of amino acids as directing agents to synthesize hierarchical silver microspheres assembled by nanosheets with well-defined morphologies, in the absence of any other surfactants or capping agents. This fabrication method avoids the absorption of macromolecules and enables clean surface on the Ag microspheres. The chemical nature of the amino acids plays a vital role in the hierarchical structure of the Ag microspheres. As found, amino acids with simple structures and 2−3 carbon atoms like alanine and glycine lead to more loosely packed Ag microspheres, and those with more complicated structures and more carbon atoms, e.g. glycine, glutamine, and asparagine, result in close-packed Ag particles assembled by thinner nanosheets. By adjusting the concentration of AgNO3 solution, size as well as the surface roughness of the Ag microspheres can be well controlled. Individual particles of the constructed hierarchical Ag microspheres with highly roughened surface can act as sensitive SERS platforms. Detection of chemical molecules and monitoring of the plasmon-driven chemical reactions have been carried out through a single particle SERS technique.

I. INTRODUCTION Nanotechnology shoulders the task of discovering new materials with tunable electronic, optical, mechanical, and transport properties required to overcome many of today’s technological challenges.1−4 Tremendous progress has been made in synthesis of nanoparticles with astonishing properties over the past two decades. However, controllable assembly of nanoparticles into periodic structures has only been achieved for a handful of systems under very specific conditions. Thus, controlled and reproducible synthesis of larger systems with well-defined nanostructures has received much attention in modern nanoscience and nanotechnology owing to their potential application in optics, electronics, catalysis, and sensing.5−7 The advantages of the complex self-assemblies, which are distinctly different from the corresponding individual nanoparticles and bulk solid, are driving the exploration of synthetic approaches to manipulate the morphologies and chemical composition of nanostructures.8−12 Upon appropriate excitation energy, these assembles with novel nanostructures can sustain large electromagnetic fields at cavities, interstitial sites, tip apexes, or void spaces that lead to surface-enhanced Raman scattering (SERS).13−17 However, for the fabrication of ultrasensitive devices, controlled synthesis of SERS substrates with so-called “hot spots” has been proved enormously difficult to accomplish due to limited understanding of fundamental mechanisms.18−20 Development of novel synthesis craft to generate metal nanoparticle assemblies with exquisite nanostructures as SERS substrates has attracted more and more attention in material © 2013 American Chemical Society

area and synthetic chemistry, especially in mild and low-cost routes. Larger ordered structures have been obtained by using the approaches of electrostatic force,21,22 selective adhesion of DNA,23 magnetic force,24−26 and capillary force.27 Unfortunately, most of methods mentioned above are related to process complexity and/or higher cost, which do not meet the synthetic requirement of mild and low cost in modern science. Moreover, the surfactants such as sodium dodecylbenzenesulfonate (SDBS),28 polyvinylpyrrolidone (PVP),29 and cetyltrimethylammonium bromide (CTAB)30 are usually employed as directing agents and/or capping agents to induce anisotropic growth of nanostructures in solution chemistry. The applied surfactants can successfully control the size and the morphology of the metal nanoparticles with roughened surface concerning electromagnetic enhancement in SERS. However, as-prepared metal structures served as SERS substrate are not sufficient to detect the analyte molecules, as they cannot be directly adsorbed on the surface of the structures due to the disturbance of polymer molecule residual on the surface even after repeated rinsing.31 Synthesis of luminescent and Raman-active 2−30 nm Ag nanoparticles (NPs) by thermal reduction of silver ions in glycine matrix has been reported, taking advantage of the solidstate matrix to control the nucleation and migration of reduced Ag atoms.32 Nearly monodisperse Ag NPs in large quantities Received: January 17, 2013 Revised: April 17, 2013 Published: April 22, 2013 10007

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III. RESULTS AND DISCUSSION Our previous work has shown that the acid introduced in synthesizing Ag nanostructures through a solution chemistry route can dramatically alter the assembly manner of the Ag nanoparticles.7 Here, very limited amount of amino acids incorporated into the reaction solution can result in Ag particles with morphologies highly dependent on the type of amino acid. As shown in Figure 1, Ag particles with uniform size and well-

have been prepared via a microwave-assisted method in an aqueous system, using basic amino acids, such as L-lysine or Larginine, as reducing agents and soluble starch as a protecting agent.33 Herein, we report a facile and effective protocol to prepare self-assembled Ag nanostructures through the addition of a small amount of amino acids in the reaction system, without the introduction of any macromolecular surfactants or capping agents. Of note is that here the amino acids added into the reaction system just function as efficient directing agents to drive the assembly of Ag NPs into complex hierarchical structures with roughened surface. Individual particles of the asprepared hierarchical Ag microspheres can be applied in a single particle SERS technique for sensitive chemical detection and monitoring plasmon-driven reactions.

II. EXPERIMENTAL SECTION Materials. AgNO3 (99.9%, Sinopharm Chemical Reagent Co., Ltd.), ascorbic acid (C6H8O6 99.8%, Tianjin Damao Chemical Reagent Factory), 4-mercaptobenzoic acid (4-MBA, 99.99%, Tokyo Chemical Industry), methylene blue (MB, Aldrich), amino acids (histidine (AR), alanine (AR), glycine (AR), glutamine (AR), and asparagine (AR), Tianjin Guangfu Fine Chemical Research Institute), and o-, m-, and paminothiophenol (J&K) were used without further purification. Amino Acid-Assisted Synthesis of Ag Nanostructures. Silver nanostructures were synthesized according to a previously reported procedure.7 In a typical synthesis, 1 mL of AgNO3 aqueous solution (1, 0.5, and 0.1 M) and 50 μL of 0.1 M amino acids (histidine, alanine, glycine, glutamine, or asparagine) were added into 10 mL of deionized water in a 25 mL beaker with a magnetic stirrer in an ice−water bath. Ten minutes later, ascorbic acid aqueous solution (1 M) was quickly injected into the vigorously stirred mixture. Color of the solution became gray or black along with a large number of silver particles produced in a few minutes. The reaction was terminated at a reaction time of 15 min by centrifugation, and the silver particles were collected. The resulting particles were repeatedly rinsed with deionized water and ethanol. Then the samples were dried in a vacuum drier to prevent the oxidation of the Ag surface. Characterization. Scanning electron microscopic (SEM) images were collected on a Hitachi S-4800 electron microscope to study the morphology and size of the Ag particles. X-ray diffraction (XRD) measurements were performed on a D8 focus Powder X-ray diffractometer (Bruker, Cu Kα radiation). The Ag particles were soaked in 4-MBA ethanol solution or MB aqueous solution for 30 min and then rinsed several times with water to remove the residuals on the surface. The resulting Ag particles were dispersed on glass substrates before the surface-enhanced Raman scattering (SERS) responses were determined. The SERS spectra were recorded on a Renishaw in Via micro-Raman spectroscopy system, using the TE air-cooled 576 × 400 CCD array in a confocal Raman system (wavelength: 633 nm). The incident laser power was kept at 2 mW, and a total accumulation time of 5 s was employed. Raman images were collected at the 1618 cm−1 of MB. Plasmon-driven chemical reactions were monitored on a single Ag particle using the 633 nm excitation laser, with a power of 0.5 mW.

Figure 1. SEM images of Ag structures prepared by the assistance of a small amount of different amino acids: (a, b) alanine, (c, d) glycine, (e, f) histidine, (g, h) glutamine, and (i, j) asparagine. The molecule formula in each row shows the corresponding amino acid applied in the synthesis process.

defined structures are obtained through our amino acid-assisted synthesis route. It is verified that the selected amino acids can be efficient directing agents to assemble the Ag nanoparticles into complex structures with highly roughened surface. With 1 M AgNO3 aqueous solution, all Ag particles produced are spherical in morphology, which are actually assembled by numerous nanosheets. A close look at these Ag particles reveals that different amino acid can affect the stack density of the nanosheets. Amino acids with simple structures and 2−3 carbon atoms like alanine (Figures 1a,b) and glycine (Figures 1c,d) lead to more loosely packed Ag particles that are about 2−3 μm in diameter, and the nanosheets that form the assemblies are 100−150 nm in thickness, while application of amino acids with more complicated structures and more carbon atoms, e.g. glycine (Figures 1e,f), glutamine (Figures 1g,h), and asparagine (Figures 1i,j), results in Ag particles with larger sizes (3−4 μm) but thinner nanosheets (50−100 nm). These Ag particles bear more close-packed structures, as compared to the Ag particles produced with the assistance of alanine and glycine. It can be seen from the magnified images that the nanosheets grow along different directions and assemble in different ways 10008

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to form the Ag microspheres. We think the morphological difference of Ag structures obtained here should be associated with the chemical nature and structural feature of the amino acids that can change the assembly manner of the Ag NPs when adsorbed on the Ag nuclei, as we found in our previous work.7 Though the Ag particles obtained in the absence of any amino acid have a smaller diameter of about 1.3 μm, the entire surface of the Ag particles is pretty smooth without any exquisite nanostructures (Figure S1). Figure 2 shows the typical X-ray diffraction (XRD) patterns of the prepared Ag particles with the assistance of five amino

Figure 3. SERS spectra of 10−6 M 4-mercaptobenzonic acid (4-MBA) recorded from the obtained assembled Ag nanostructures with the assistance of (a) alanine, (b) glycine, (c) histidine, (d) glutamine, and (e) asparagine.

binding as compared to the more close packed Ag structures. According to eq 1 I c EF = SERS R IR cSERS (1) where I SERS and c SERS are the Raman intensity and concentration measured from the SERS sample (on Ag) and IR and cR are the intensity and concentration for the reference sample, we have calculated the enhancement factor (EF) of these Ag microspheres with different surface nanotextures. The Ag microspheres with loosely packed nanosheets have an EF of ∼106, and EF of those with closely packed nanosheets is ∼107. This result again confirms that Ag microspheres comprised of closely packed nanosheets should have larger surface areas for molecule binding and thus better SERS responses. Of note is that no Raman features of the applied amino acids are detected for all the Ag particles, an evidence showing that the amino acids used to direct Ag growth have been completely removed from the surface of Ag assemblies by repeatedly rinsing with deionized water and ethanol. These assembled Ag nanostructures obtained here can be readily used as efficient platforms which avoid the disturbance of reactants and enable high sensitivity. In order to control the size and morphology of the Ag particles, we tried the synthesis procedure by tuning the concentration of AgNO3 solution. Considering the morphology and uniformity of Ag particles, we choose both glutamine and asparagine as objects of study during the Ag growth process. Figures 4a−f show the SEM images of the Ag particles produced by the assistance of glutamine with different concentrations of AgNO3 solution. With 0.1 M AgNO3 (Figures 4a,b), the obtained Ag particles are ∼1.3 μm in size with very smooth surface, resembling the morphology of the Ag particles obtained without amino acids (Figure S1). By increasing the AgNO3 concentration to 0.5 M, Ag particles with obvious surface nanotextures and larger diameter of ∼1.7 μm were produced (Figures 4c, d). From Figures 4e,f, one can see that Ag particles with even increased size (∼3 μm) and surface roughness are obtained when the concentration of AgNO3 solution is increased to 1.0 M. We think it can be rationalized by the fact that, with low concentration of AgNO3, most Ag+ ions are consumed during the nucleation process, and surface growth into roughened nanotextures will be very

Figure 2. XRD patterns of the prepared Ag particles with the assistance of (a) alanine, (b) glycine, (c) histidine, (d) glutamine, and (e) asparagine.

acids, where the diffraction peaks can be well indexed to the (111), (200), (220), (311), and (222) crystal planes of the facecentered cubic (fcc) Ag crystals. The sharp diffraction peaks imply that the as-prepared Ag assemblies with the assistance of amino acids are all well-crystallized by using ascorbic acid as reducing agent. The intensity ratio of the diffraction peaks is almost identical to that of the particles prepared without amino acids (Figure S2), which indicates that the addition of amino acids into the reaction system would not induce the anisotropic growth along a certain crystal plane. However, it is immensely useful to direct the Ag nanoparticles to assemble into hierarchical structures. We investigated the SERS properties of the assembled Ag nanostructures with the assistance of various amino acids by using 4-mercaptobenzonic acid (4-MBA) as the analyte molecule, which strongly adsorbs on the surface of the Ag hierarchical structures by forming an Ag−S bond. The Raman spectrum of 4-MBA is dominated by the ν8a (∼1590 cm−1) and ν12 (∼1080 cm−1) aromatic ring vibrations; other weak bands at ∼1150 and ∼1180 cm−1 are attributed to the C−H deformation modes.7,34 As shown in Figure 3, well-resolved Raman spectra can be collected at a 4-MBA concentration of 10−6 M, which corresponds to ∼1 ppm. In this regard, it can be interpreted that the Ag assemblies with well-defined nanostructures directed by different amino acids couple strongly to the external electromagnetic field. This coupling, which is mediated by surface plasmon resonance, highly depends on the size and morphology of the nanocrystals.2 However, a careful analysis of the Raman spectra indicates that the more loosely packed Ag particles produced with the assistance of alanine and glycine have relatively lower Raman intensities, which should be due to less SERS-active sites and limited surface areas for molecule 10009

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Figure 4. SEM images of Ag particles prepared with the direction of a small amount of glutamine at different concentration of AgNO3 solution: (a, b) 0.1 M, (c, d) 0.5 M, (e, f) 1.0 M. XRD patterns (g) and SERS spectra of 10−6 M 4-MBA (h) of Ag particles obtained with different concentrations of AgNO3 solution.

limited. As a comparison, assembled growth at the nuclei surface will be dominant after nuclei formation with a higher concentration of AgNO3, leading to Ag particles with larger sizes and obvious hierarchical structures. It can be seen from the XRD patterns (Figure 4g) that the crystallinity of the Ag particles prepared from various AgNO3 concentrations seems getting better with increased concentration, as the diffraction intensity is much enhanced, while the peak width is getting smaller with increased concentration, agreeing well with the size increase of the as-obtained Ag particles. It is of no surprise that higher SERS sensitivity can be obtained from the Ag particles with obvious surface nanotextures, although with larger particle sizes (Figure 4h). Ag particles prepared from 0.1 M AgNO3 with smaller size show very limited Raman signals toward 10−6 M 4-MBA, mainly due to the smooth surface and limited surface area for molecule binding. As the surface nanostructure gets more prominent, enhanced Raman intensity is evidenced. With asparagine present in the reaction solution, similar results were obtained by tuning the concentration of AgNO3 solution (Figures S3− S5). Importantly, those hierarchical Ag microspheres with roughened surface can allow one single particle acting as reproducible SERS substrates under the confocal Raman spectroscopy for sensing and imaging applications. As molecules with thiol groups can strongly bind the Ag surface through forming Ag−S covalent bonds, here we immersed the assembled Ag nanostructures prepared with the assistance of glutamine in methylene blue (MB) solutions of different concentrations and then measured the Raman spectra by focusing the laser beam on one single Ag particle (Figure 5a). The Raman spectrum of MB is dominated by ν(C−C) ring stretching at 1618 cm−1, α(C−H) in-plane ring deformation at 1398 cm−1, β(C−H) in-plane bending at 768 cm−1, and δ(C− N−C) skeletal deformation at 495 and 445 cm−1.35 As can be seen, it is able to detect MB molecules at a concentration as low as 10−8 M on a single Ag particle, but unfortunately noise signals will be comparable to MB signals when even lower concentrations are applied. As can be seen from the Raman images (Figures 5b−f) taken from the peak corresponding to the ν(C−C) ring stretching at 1618 cm−1, the SERS hot spots typically reside at the center of a single Ag microsphere, which

Figure 5. (a) SERS spectra of methylene blue (MB) at different concentrations recorded on one single Ag particle from the assembled Ag nanostructures prepared with the assistance of glutamine. Raman images taken from the 1618 cm−1 peak of MB at different concentrations: (b) 10−4, (c) 10−5, (d) 10−6, (e) 10−7, and (f) 10−8 M. Scale bar: 5 μm.

agrees well with the structure of the Ag particles prepared with the assistance of glutamine (see Figure 1h). Nevertheless, this result shows that one single particle of the assembled Ag nanostructures can be readily used a SERS tag to detect chemical molecules at low concentrations and for imaging purposes, as the roughened surface can act as a collector and concentrator of the target molecules. Recently, by employing the “hot” electrons from surface plasmon decay, plasmon-driven chemical reactions have attracted great attention because it opens up a new pathway for studying the chemical reactions on SERS-active catalysts.17,36−39 Our single particle SERS technique can also be applied to monitor the plasmon-driven chemical reactions as the hierarchical Ag microspheres with roughened surface can serve as ideal SERS tags for studying such reactions by measuring SERS spectra and Raman images on one single particle under a confocal microscope Raman system. Our previous study showed a nice control over the reaction of pnitrothiophenol (pNTP) dimerizing into p,p′-dimercaptoazobenzene (DMAB) by tuning the laser wavelength and power.40 Here the plasmon-driven reaction of o-, m-, and p-aminothiophenol (oATP, mATP, and pATP) has been studied by focusing the laser beam on a single Ag particle (Scheme 1). It is surprising that oATP and mATP have very weak Raman signals on Ag particles, presumably due to their structure features that are in lack of Raman vibration, and it is not able to see their dimerization reactions under laser excitation (Figures S6 and S7). This may indicate that the position of the −NH2 group can dramatically influence such plasmon-driven dimerization reactions. Nevertheless, the dimerization of pATP can be easily tracked by using our single particle SERS technique. Figure 6 shows the time-dependent SERS spectra of pATP under continuous excitation by the 633 nm laser, with a power 10010

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glycine lead to more loosely packed 2−3 μm Ag particles with relatively smaller surface areas. Application of amino acids with more complicated structures and more carbon atoms, e.g., glycine, glutamine, and asparagine, results in Ag particles with larger sizes (3−4 μm) but close-packed thinner nanosheets that form the Ag microspheres. However, the addition of amino acids cannot induce anisotropic growth but just direct the Ag nanoparticles assemble into complex structures. By adjusting the concentration of AgNO3 solution, size as well as the surface roughness of the Ag particles can be well controlled. Asprepared Ag particles and even one single particle can be readily used as SERS platforms for sensing and imaging applications due to their novel surface nanostructures. Particularly, plasmon-driven chemical reaction of p-aminothiophenol dimerizing into p,p′-dimercaptoazobenzene has been successfully monitored by our single particle SERS technique.

Scheme 1. Plasmon-Driven Chemical Reaction of pAminothiophenol (pATP) Dimerizing into p,p′Dimercaptoazobenzene (DMAB) Monitored by Single Particle Surface-Enhanced Raman Spectroscopya

a

The Ag particle used here is from the same batch as that used for detecting MB in Figure 5.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images, SERS spectra, and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.X.); [email protected] (X.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.X. thanks the support from the China Postdoctor Fund, NSFC (No. 21203045, 21101041, 21003029, 21071037, and 91122002), Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2010065 and 2011017, and HIT.BRETIII. 201223), and Director’s Postdoctoral Fellow from LANL.

Figure 6. Time-dependent SERS spectra of p-aminothiophenol (pATP) under continuous exposure to 633 nm laser. The spectra were collected from a single Ag particle, with an integration time of 2 s and a laser power of 0.5 mW.

of 0.5 mW. The Raman spectrum of pATP is dominated by the bands at 1076, 1180, 1486, and 1588 cm−1 attributed to the a1 modes.37 As can be seen, the intensities of the bands at 1380 and 1440 cm−1 due to ν(NN) and 1140 cm−1 due to β(CH) gradually increase with exposure time, indicating the conversion of pATP to DMAB. As there is no spectral features of the −NH2 group of pATP, one cannot easily tell the complete conversion of this reaction. In the plasmon-driven conversion of pNTP to DMAB, the disappearance of the −NO2 group could be a signal of reaction completion. Anyway, here we have demonstrated again the ease of using the single particle SERS technique to monitor the plasmon-driven chemical reactions. It should be pointed out that the hot electrons generated from surface plasmon decay have been regarded as the source that induces the chemical reactions of the adsorbed molecules on the metal surface.39 However, conversion of pATP into DMAB is actually an oxidation reaction, and thus the underlying reaction mechanism under such a reductive environment needs to be interpreted in combination with other techniques.

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REFERENCES

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IV. CONCLUSIONS In summary, we have provided a facile yet efficient synthetic technique to fabricate homogeneous Ag particles assembled by nanosheets with well-defined nanostructures by the assistance of amino acids. The chemical nature of the amino acids plays a vital role in the morphology of Ag particles. Amino acids with simple structures and 2−3 carbon atoms like alanine and 10011

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