Silver Nanostars with High SERS Performance - The Journal of

Article pubs.acs.org/JPCC

Silver Nanostars with High SERS Performance Adianez Garcia-Leis, Jose Vicente Garcia-Ramos, and Santiago Sanchez-Cortes* Instituto de Estructura de la Materia, IEM-CSIC, Serrano 121, 28006 Madrid, Spain S Supporting Information *

ABSTRACT: We report in this work the fabrication, for the first time, of silver nanostars (AgNS) by a simple new method consisting of the chemical reduction of Ag+ by neutral hydroxylamine, followed by a capping−reduction process induced by citrate. TEM and SEM were employed to study the morphology of the resulting nanoparticles, which exhibit a star-shaped morphology with a central particle provided with several arms or protuberances with low sharpness in the vertices. Dark field microscopy was employed to study the scattering emission of individual nanostars indicating the increasing presence of nanoparticles with scattering emission toward the red region. AgNS displayed a high performance in surface-enhanced Raman scattering (SERS) applications. The effectiveness of these nanoparticles was probed by using the drug probenecid, leading to intense SERS spectra without the addition of aggregation agents.

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INTRODUCTION The interest in plasmonic metal nanoparticles (NPs) has grown in recent years due to the large list of applications in many fields such as the environment, medicine, chemistry, and optics thanks to their great potentiality in chemical detection, clinical diagnosis, heterogeneous catalysis, and many other applications.1 These systems are very sensitive due to the localized surface plasmon resonances (LSPRs) in the surface of metal plasmonic NPs, which can lead to a strong enhancement of the electromagnetic (EM) field on the interface.2 The application of NPs in surface-enhanced Raman scattering (SERS) has been well-known since the very beginning of the appearance of this technique. Spherical NPs (SNPs) were previously employed to get large intensifications in SERS.3 However, the Raman enhancement induced by SNPs is relatively low. To increase the SERS performance of these systems, an aggregation of the colloidal suspensions must be carried out, since a huge intensification of the field can be induced in interparticle gaps or on the surface of large aggregates.4 Obviously, this aggregation is a serious drawback of the SERS application in analytical quantitative applications, since this process implies a lowering of the experiment’s reproducibility. Another strategy that can be followed to get large SERS intensification is the preparation of NPs with a large LSPR that do not need aggregation. Anisotropic metal NPs with a large variety of sizes and shapes have been recently fabricated displaying good SERS properties. In this context, star-shaped NPs or metal nanostars (NS) have shown extraordinary properties in the intensification of the EM field, with promising applications in bioimaging and detection.1c,5 Some examples of the applications of these NS in SERS have been reported.5b Surprisingly, the main part of works published up to now about star-shaped nanosystems were accomplished by using NS made of Au (AuNS).6 © 2013 American Chemical Society

However, silver nanostars have not been reported so far except for the nanobranched NPs synthesized by Camargo et al.7 The fabrication of silver nanostars (AgNS) should be of high interest in spectroscopy and, therefore, in the optical detection field, due to the better optical properties of this metal in comparison to gold, taking into account the LSPR properties of each metal.8 In general, AgNPs have a broader EM region of activity, ranging from blue to the near-IR (NIR), in contrast to AuNPs, which only show LSPR activity in the red-NIR region. In addition, AgNPs display higher SERS enhancement factors (EF) in comparison to gold ones.9 Furthermore, AuNS prepared so far generally require the use of surfactants and other compounds that remain adsorbed on the surface, thus limiting seriously the application in SERS spectroscopy of these systems. Furthermore, these NS have been successfully applied in SERS but only in the detection of analytes which also have a strong response on spherical NPs.10 This work was aimed at the fabrication of AgNS by simple methods with high effectiveness in SERS and without the use of strong surfactants. To accomplish this task, neutral hydroxylamine (HA) in aqueous solution was employed as primary chemical reductor. This compound affords the necessary conditions to induce the growth of spiked nanoparticles. Afterward, the reduction was accelerated and completed by the addition of citrate. This method avoids the use of other strong capping surfactants that may prevent the access of analytes to metallic surfaces, and their detection by SERS, even if the resulting AgNS were finally covered by citrate ions to increase their stability in suspension. Citrate ion is a very soft capping Received: February 19, 2013 Revised: March 26, 2013 Published: March 26, 2013 7791

dx.doi.org/10.1021/jp401737y | J. Phys. Chem. C 2013, 117, 7791−7795

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plasmon absorption spectra. Samples were placed in quartz cells of 1 cm optical path, after dilution to 50% in Milli-Q water (v/ v), except for sample C, which was not diluted. The Fourier transform (FT) Raman normal spectrum of the pure solid was registered using a Bruker RFS/100S FT-Raman instrument, equipped with a Nd:YAG laser beam at 1064 nm and a Ge detector cooled with liquid nitrogen. SERS spectra were collected on a Renishaw Raman InVia spectrometer equipped with an electrically cooled CCD camera. Samples were excited by using the 532 nm laser line provided by a frequency-doubled Nd:YAG laser and a power of 2.5 mW at the sample. The spectral resolution was set in all cases to 2 cm−1. SERS spectra were registered with a total acquisition of 10 s for each SERS spectrum and consisted of only one scan. Dark field images and scattering spectra were registered with a CytoViva (Auburn, AL) instrument, attached to an Olympus microscope (EDFM). The system consisted of a CytoViva 150 dark field condenser in place of the microscope’s original condenser, attached via a fiber optic light guide to a Solarc 24 W metal halide light source (Welch Allyn, Skaneateles Falls, NY). A 100× immersion oil objective provided with an iris (Olympus UPlanAPO) was employed. Transmission electron microscopy (TEM) images were taken using a JEOL JEM-1011 with an acceleration voltage of 100 kV. Scanning electron microscopy (SEM) images were obtained using a Hitachi SU-6600, 30 kV voltage acceleration, coupled to an energy dispersive X-ray (EDX) detector.

compound which is easily removable from the surface in the presence of other adsorbates manifesting a stronger affinity for the silver surface. Different experimental conditions were investigated to understand their effect on the morphology of resulting particles. Finally, the SERS effectiveness of these substrates was investigated by using probenecid (PB), a sulfamide diuretic drug normally tested in antidoping analysis, which is a molecule with a relatively low affinity to bind the Ag surface and that represents an actual challenge for the application of the fabricated AgNS.

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EXPERIMENTAL METHODS AgNO3, trisodium citrate, and absolute ethanol (all of analytical grade) were purchased from Merck. PB (≥98%), hydroxylamine hydrochloride, and hydroxylamine solution (50 w/w in water) were purchased from Aldrich. All solutions were prepared in Milli-Q water. Fabrication of Silver Nanoparticles. Colloidal suspensions of AgNS were prepared by chemical reduction of Ag+ in two steps and using as reducing agents neutral hydroxylamine (HA) in a first stage and citrate (CIT) in a second step. Several preparation methods were assayed in this work, giving rise to samples A−D. These methods consisted of the use of different relative concentrations of Ag+, HA, and CIT as indicated in Table 1. Table 1. Concentrations of Reagents Employed for the Preparation of Samples A−E sample A sample B [AgNO3], mM [HA], mM [CIT], mM pH

0.89 3.0 0.41 5.5

0.089 3.0 0.41 6.5

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sample C sample D sample Ea 0.89 1.5 0.41 8.0

0.89 3.0 0 5.0

RESULTS AND DISCUSSION Figure 1, upper panel, shows TEM and SEM images of AgNS contained in sample A. This sample includes star-shaped nanoparticles with the best properties from the morphological and SERS effectiveness points of view. TEM images of NPs obtained by all methods are shown in Figure S1 (Supporting

0.89 3.0b 0 6.5

a

For the preparation of this sample, hydroxylamine hydrochloride (HA·HCl) was used. b[HA·HCl].

For the sake of brevity, we only describe here method A, employed to fabricate AgNS of sample A: 500 μL of 6 × 10−2 M HA was mixed with 500 μL of NaOH (0.05 M). Afterward, 9 mL of 10−3 M AgNO3 was added dropwise to the first solution under agitation. The suspension became brown, as indicated in Figure 2D. After 5 min, 100 μL of 4.13 × 10−2 M (1%, w/v) trisodium citrate was added to the mixture. The final suspension was shaken for 15 min before measuring the pH (Table 1), showing a dark gray color (Figure 2A). Sample E, employed as a control, was prepared by reduction with HA hydrochloride according to the method reported by Leopold and Lendl.11 The latter sample does not contain nanostars, but contains mainly spherical or spheroidal NPs as reported elsewhere.12 Preparation of Samples for SERS Spectra. A stock solution of PB in ethanol at 0.2 M concentration was prepared. Further dilutions were prepared in Milli-Q water. An aliquot (typically 1 μL) of the aqueous PB solution at adequate concentration was then added to 1 mL of the colloidal AgNPs, without any activation, to get the desired final PB concentration. Samples for SERS spectra were measured in a glass vial focusing the laser beam inside. Instruments. Extinction spectra of colloids were recorded on a Shimadzu 3600 spectrometer equipped with a photomultiplier tube for light detection in the UV−visible range, and an InGaAs detector for the NIR was employed to obtain the

Figure 1. Micrographs of AgNS obtained by (I and II) TEM and (III and IV) SEM from sample A. Bottom: growing sequence deduced from different Ag nanoparticles observed in TEM images of (a−c) sample D nanoparticles and sample A after (d) 24 and (e) 48 h of reduction with citrate. 7792

dx.doi.org/10.1021/jp401737y | J. Phys. Chem. C 2013, 117, 7791−7795

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Information). AgNS of sample A are characterized by the existence of several arms (ranging from 7 to 12) which radiate from a core. The number of arms varies from NP to NP, with eight being the average number. Sometimes these arms are branched as can be seen in Figure 1II. AgNS of sample A have an average diameter of 300−400 nm. In general, the tips of the observed arms display a low sharpness. The analysis of the control sample D, i.e., in the absence of citrate, reveals that the reduction with neutral hydroxylamine leads to the appearance of NPs bearing different morphologies (i) highly faceted Ag NPs (Figure 1a, bottom panel) and (ii) small star-shaped NPs (Figure 1b,c, bottom panel)although with a much smaller size than those NPs observed in sample A. This result suggests that the reduction with HA already induces the formation of AgNS, but through a very slow process at room temperature and pH 5.5, leading to the formation of faceted NPs at a first stage. These NPs can act as seeds for the subsequent growth induced by HA, which can proceed with the reduction of Ag+ ions and their deposition on the planes of faceted NPs leading to the growth of the observed primary protuberances (Figure 1b,c, bottom panel). Thus, the reduction of HA is enough to induce the formation of small AgNS. The posterior addition of citrate in the preparation of sample A highly accelerates the process, leading to the growth of very long arms (Figure 1d,e, bottom). The length of these arms can be modulated by varying the relative amounts of Ag+, HA, and citrate. If the amount of HA is decreased in relation to Ag+ (sample C), the number of AgNS is lowered due to the formation of a large number of growing centers, which grow following a spherical morphology (Figure S1 in the Supporting Information). In contrast, if the amount of Ag+ is decreased (sample B), the length of arms in the resulting NS is shorter, due to the lower amount of silver atoms available in the mixture. Time also seems to play a key role in the reduction process. In fact, TEM image obtained 24 h after the addition of citrate (Figure 1d, bottom) shows AgNS with shorter arms that those obtained 48 h after addition (Figure 1e, bottom). The study of all these effects (different reduction steps with HA, stimulated reduction with citrate, and reduction time) leads us to propose the growing sequence indicated in Figure 1 (bottom), which implies the formation of initial faceted NPs and their gradual growing to render the final spiked AgNS. The stability in time of sample A was monitored with the extinction spectra after addition of citrate. Figure S2 in the Supporting Information reveals that the main changes in the extinction spectrum occur in the first 48 h. Afterward, the suspension only undergoes a slight intensity decrease due to a possible precipitation of the larger NPs. Figure 2 shows the extinction spectra of colloidal AgNP suspensions corresponding to samples A−D. As can be seen, samples A and B give rise to similar spectra displaying two strong peaks at 376 and 369 nm, likely due to bulk plasmon resonances,13 and an extinction background at longer wavelengths where weak maxima are distinguished at 750 and 618 nm for samples A and B, respectively. The large extinction background can be attributed to the absorption and scattering emissions produced by the different morphologies of all the existing NPs, integrated by AgNS bearing different numbers of arms and having different tip sharpnesses. The mixture of all these factors leads to very different LSPR in the suspension since a wide range of wavelengths in the visible and near-IR can be covered with various NS shapes.14

Figure 2. Extinction spectra and images of samples (a) A, (b) B, (c) C, and (d) D. Inset: pictures of colloids obtained by methods A, B, C, and D.

In order to get more insight about the plasmon resonances of these NPs, the extinction spectra from single nanoparticles were measured by dark field scattering. For the sake of brevity, dark field images of samples A and E, together with the most frequent scattering spectra from the individual NPs and the relative distribution of NPs, are shown in Figure 3. Dark field images and individual plasmon spectra from the rest of the samples are shown in Figure S1 in the Supporting Information. As can be seen in Figure 3, sample A displays scattering plasmon resonances at 475 (blue), 550 (green), 640 (orange, with a shoulder at 510 nm), 650 (red), and 670 nm (pink, with two shoulders at 498 and 595 nm). The blue scattering spectrum can be assigned to the emission of spherical NPs, while the spectra observed at higher wavelengths are attributed to aggregates and nanostars. The scattering spectra peaking at different wavelength values can be attributed to the existence of AgNS with different numbers of arms.14b On the other hand, sample E shows maxima at 460 (blue), 545 (green), 597 (orange), 620 (yellow, with a shoulder at 550 nm), and 650 nm (red). The blue spectrum can be again assigned to spherical AgNPs, while the spectra observed at higher wavelengths are attributed to aggregates (dimers, trimers, and bigger aggregates) existing in the TEM sample because of the aggregation induced during the drying process. Sample E displays a higher percentage (see inset distribution of NPs in Figure 3b) of blue scattering spectra as corresponds to the more frequent presence of SNPs or small aggregates in the sample. On the contrary, sample A shows a higher content of scattering spectra toward the red (inset in Figure 3a), mainly at 600−700 nm, attributed to the AgNS observed in the TEM images. Finally, the SERS effectiveness of AgNS fabricated by the A method was monitored by using probenecid (PB) as probe molecule (Figure 4). This drug contains a benzoic acid group which is ionized to benzoate at the pH of the colloid (pH 5.5). This fact highly prevents the approach of PB to metal surface in general, thus leading to weak SERS spectra. Therefore, we have employed this drug to actually test the SERS activity of the 7793

dx.doi.org/10.1021/jp401737y | J. Phys. Chem. C 2013, 117, 7791−7795

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AgNS reported here. Figure 4b shows the SERS spectrum of PB on AgNS of sample A, in comparison to the SERS spectrum on SNPs of sample E (Figure 4c), corresponding to the usual method reported by Leopold and Lendl, which implies the use of hydroxylamine hydrochloride as reducing agent.11 Intense bands can be seen in the SERS spectrum of PB on AgNS of sample A, this without aggregating the colloidal suspension by the addition of saline aggregation agents. This is possible thanks to the large intensification of electromagnetic field occurring on these NPs on the NS tips, but also in spaces between arms.13,14 The detailed analysis of the SERS spectra indicated the presence of bands at 1374 and 952 cm−1, assigned to the ν(COO−) and ν(C−COO−) motions, respectively (Figure 4b), thus suggesting that the drug adsorption on AgNS takes place through the carboxylate group as shown in the inset scheme of Figure 4. This is connected to the weakening of the band seen at 1638 cm−1 in the Raman spectrurm of the solid (Figure 4a), attributed to a combination of ν(CC) and ν(CO) vibrations in the benzoic acid, due to the corresponding ionization of the COOH group. The limit of detection deduced from the adsorption isotherms of PB on AgNS (Figure S3 in the Supporting Information) was 51.3 ng/mL (see Supporting Information for more details), which is of the same order as those obtained by HPLC.15 The enhancement factor calculated for PB at a final concentration of 5 × 10−7 M, and when exciting at 532 nm, was 1.72 × 105 (Supporting Information). This is indeed a relatively high factor taking into account that no aggregating agent was added to the suspension. In conclusion, method A reported in this paper gives rise to silver star-shaped nanoparticles with good plasmonic properties to afford a large SERS intensification. These AgNS exhibit a core from which several arms are expanded, with the length of these arms subjected to modification by controlling the concentration of reagents and the reduction time. This new kind of NPs demonstrated a high SERS performance and can be successfully applied in SERS spectroscopy because of two reasons: (a) they exhibit clean adsorption surfaces, free from the strong surfactants usually employed to fabricate other types of nanostars, thus allowing the easy adsorption of analytes, and (b) the aggregation is not necessary because of the large intensification provided by their special morphology. As concerns the spectroscopic applications of these NPs, it was demonstrated that they can be successfully applied in a broader region of the electromagnetic spectrum, since they displayed plasmon resonances ranging from the blue region toward the red-NIR region.

Figure 3. Dark field scattering spectra observed in samples (a) A and (b) E. Inset: dark field and TEM images of the corresponding samples with the percent distribution (bars) of the number of nanoparticles producing the scattering emission of the corresponding color.

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ASSOCIATED CONTENT

S Supporting Information *

Dark field images, single particle scattering emission spectra, maps of dark field images on the basis of the scattering spectra, distribution of the metal NPs giving rise to the main scattering spectra, and TEM images of all samples. Stability study of sample A by analysis of extinction spectra. Experiment conducted for the detection of probenecid and the calculation of the limit of detection and the enhancement factor. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Figure 4. (a) Raman spectrum of PB in solid state exciting at 1064 nm and (b, c) SERS spectra of PB (10−4 M) on (b) AgNS of sample A and (c) spherical AgNPs of sample E exciting at 532 nm. Inset: scheme of the adsorption of PB on Ag deduced from the analysis of the SERS spectrum.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 7794

dx.doi.org/10.1021/jp401737y | J. Phys. Chem. C 2013, 117, 7791−7795

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Author Contributions

Colloids at Room Temperature by Reduction of Silver Nitrate with Hydroxylamine Hydrochloride. J. Phys. Chem. B 2003, 107, 5723− 5727. (12) Cañamares, M. V.; Garcia-Ramos, J. V.; Gomez-Varga, J. D.; Domingo, C.; Sanchez-Cortes, S. Comparative Study of the Morphology, Aggregation, Adherence to Glass, and Surface-Enhanced Raman Scattering Activity of Silver Nanoparticles Prepared by Chemical Reduction of Ag+ Using Citrate and Hydroxylamine. Langmuir 2005, 21, 8546−8553. (13) Giannini, V.; Rodríguez-Oliveros, R.; Sánchez-Gil, J. A. Surface Plasmon Resonances of Metallic Nanostars/Nanoflowers for SurfaceEnhanced Raman Scattering. Plasmonics 2010, 5, 99−104. (14) (a) Rodríguez-Oliveros, R.; Sánchez-Gil, J. A. Gold Nanostars as Thermoplasmonic Nanoparticles for Optical Heating. Opt. Express 2012, 20, 621−626. (b) Ma, W. Y.; Yang, H.; Hilton, J. P.; Lin, Q.; Liu, J. Y.; Huang, L. X.; Yao, J. A Numerical Investigation of the Effect of Vertex Geometry on Localized Surface Plasmon Resonance of Nanostructures. Opt. Express 2010, 18, 843−853. (15) Zhang, C.; Wang, L.; Yang, W.; Wang, X. S.; Fawcett, J. P.; Sun, Y. T.; Gu, J. K. Validated LC-MS/MS Assay for the Determination of Felbinac: Application to a Preclinical Pharmacokinetics Study of Felbinac Trometamol Injection in Rat. J. Pharm. Biomed. Anal. 2009, 50, 41−45.

The paper was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Spanish Ministerio de Economiá y Competitividad (MINECO, Grant FIS201015405) and Comunidad de Madrid through the MICROSERES II network (Grant S2009/TIC-1476). A.G.-L. acknowledges CSIC and FSE 2007−2013 for a JAE-CSIC predoctoral grant. Srdja Drakulic is acknowledged for TEM measurements at CNB, CSIC, Madrid.

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