Chemically attached gold nanoparticle–carbon nanotube hybrids for highly sensitive SERS substrate

Chemical Physics Letters 512 (2011) 237–242

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Chemically attached gold nanoparticle–carbon nanotube hybrids for highly sensitive SERS substrate Lule Beqa, Anant Kumar Singh, Zheng Fan, Dulal Senapati, Paresh Chandra Ray ⇑ Department of Chemistry, Jackson State University, Jackson, MS, USA

a r t i c l e

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Article history: Received 3 May 2011 In final form 12 July 2011 Available online 20 July 2011

a b s t r a c t Surface-enhanced Raman spectroscopy (SERS) has been shown as one of the most powerful analytical tool with high sensitivity. In this manuscript, we report the chemical design of SERS substrate, based on gold nanoparticles of different shapes-decorated with carbon nanotube with an enhancement factor of 7.5  1010. Shape dependent result shows that popcorn shape gold nanoparticle decorated SWCNT is the best choice for SERS substrate due to the existence of ‘lightning rod effect’ through several sharp edges or corners. Our results provide a good approach to develop highly sensitive SERS substrates and can help to improve the fundamental understanding of SERS phenomena. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Normal Raman scattering is an inelastic scattering process and due to its second order dipole transition nature, the normal Raman signal is usually very weak [1–6]. Almost 33 years ago, it was first reported that when molecules were adsorbed onto roughened surfaces or metal nanoparticles, the Raman signals are enhanced by several orders of magnitude, which is known as surface-enhanced Raman spectroscopy (SERS) [1]. Recently SERS has been identified to be one of the most powerful and versatile analytical tool, with detection limits down to the single molecule due to the chemical and electromagnetic enhancement contributions [7–18]. Electromagnetic enhancement effect is believed to be due to the excitation of the surface plasmon resonance (SPR) on the metal surface. Gold nanoparticles of different shapes and single wall carbon nanotubes (SWCNTs) are the two most common nanometer building blocks used for material applications recently [7–24]. Because of their outstanding mechanical, optical, electrical, and thermal properties, SWCNTs have been thought to be the forefront one dimensional nanomaterial for the applications in energy storage and conversion, catalysis, sensing, medical diagnosis, and treatment [8–10,19–29]. But due to the presence of strong van der Waals interactions that tightly hold them together, forming bundles, make SWCNTs to be insoluble in all solvents. As a result, modification of SWCNTs using chemical functionalization is necessary to enhance solubility and produce novel hybrid materials which are potentially suitable for different applications [8–10,19–29]. The combination of SWCNTs with metal nanostructures are now believed to be novel organic–inorganic hybrid architectures with desirable functionalities and applications [8–10,22–28]. Due to ⇑ Corresponding author. Fax: +1 601 979 3674. E-mail address: [email protected] (P.C. Ray). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.07.037

the established synthetic protocols for the controlled preparation of gold colloidal nanostructures and the presence of unique optical properties, gold nanomaterials have enormous potential in biology and medicine after functionalization with appropriate surface moieties [11–14,29–35]. Using these unique properties of gold nanoparticles of different shapes and carbon nanostructure, in this Letter we report design of novel hybrid nanomaterials based on SWCNTs attached to gold nanoparticle (SWCNT/GNP) of different shapes as SERS substrate with very high sensitivity. Due to the presence of huge surface area and very high aspect ratio, SWCNTs are selected as useful templates where the controlled attachment of gold nanoparticles of different shape can be achieved for best enhancement of SERS signal. In SWCNT/GNP hybrid nanomaterials, metal nanoparticles are in close contact which generates ‘hot’ sites to enhance the local E-fields as well as the Raman signal. 2. Experimental section 2.1. Materials and experiments Hydrogen tetrachloroaurate (HAuCl43H2O), NaBH4, sodium citrate, silver nitrate, ascorbic acid, hexa-decyl trimethyl-ammonium bromide (CTAB), and SWCNT were purchased from Sigma–Aldrich and used without further purification. 2.2. Synthesis of popcorn shape gold nanoparticle Our gold nano-popcorn synthesis was achieved through a twostep process, as we have reported recently [13]. In first step, very small, reasonably uniform, spherical seed particles are generated using tri-sodium citrate and sodium borohydride. In the second step, we have used ascorbic acid as weak reductant as well as CTAB as shape templating agent. JEM-2100F transmission electron

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microscope (TEM) and UV–visible absorption spectrum were used to characterize the nanoparticles. Concentration of popcorn shape gold nanoparticle was determined using extinction coefficient 4.6  109 M1 cm1, as we reported recently [13]. 2.3. Synthesis and characterization of gold nanorods Gold nanorods were synthesized using a seed-mediated, surfactant-assisted growth method in a two-step procedure, as we reported before [30, 31]. Colloidal gold seeds (1.5 nm diameter) were first prepared by mixing aqueous solutions of hexa-decylcetyl trimethyl-ammonium bromide and hydrogen tetrachloroaurate(III) hydrate. An aqueous solution of sodium borohydride) was then added. The colloidal gold seeds were then injected into an aqueous growth solution of CTAB (0.1 M, 4.75 mL), silver nitrate (0.01 M, varying amounts of silver between 20 and 120 lL depending on desired nanorod aspect ratio), hydrogen tetrachloroaurate (III) hydrate (0.01 M, 0.2 mL), and ascorbic acid (0.1 M, 0.032 mL). Nanorods were purified by several cycles of suspension in ultrapure water, followed by centrifugation. Nanorods were isolated in the precipitate, and excess CTAB was removed in the supernatant. Nanorods were characterized by TEM and absorption spectroscopy, as reported before. 2.4. Fourier transform infrared (FT-IR) spectroscopy Infrared spectroscopy (IR) was used for characterization of chemical bonds before and after modifications if new bonds were formed. Transmittance spectra were obtained using a Nicolet Nexus 670 FT-IR equipped with a DTGS detector. 2.5. Surface-enhanced Raman spectroscopy (SERS) For SERS experiment, we have used a portable SERS probe, as we have reported recently [12,13]. In brief, we have used a continuous wavelength DPSS laser from laser glow technology (LUD-670) operating at 670 nm, as an excitation light source. In Photonics 670 nm Raman fiber optic probe has been used for excitation and data collection. It is a combination of 90 lm excitation fiber and 200 lm collection fiber with filtering and steering micro-optics. A miniaturized QE65000 scientific-grade spectrometer from Ocean Optics has been used as a Raman detector, with spectral response range 220–3600 cm1. It is equipped with TE cooled 2048 pixel CCD and interfaced to computer via a USB port. At the end, the Raman spectrum was collected with Ocean Optics data acquisition SpectraSuite spectroscopy software. 2.6. Results and discussions Hybrid SWCNT attached gold nanoparticle with different shapes were synthesized using multistep process as shown in Figure 1. In the first step, chemical functionalization of SWCNT tips was performed mainly on the basis of oxidative treatments using concentrated nitric acid [20–27]. For carboxylation, single walled carbon nanotube (SWCNT 200) 0.013 g was dispersed to 20 mL of HNO3 and ultrasonicated at 50 °C at power 20 W for 3 h. Oxidation process yields opened tubes with carboxylic acid functionality at both the sidewall and the tube endings [20–27]. After that, the functionalized carbon nanotubes were rinsed out through the PTFE vacuum filter till the washed off reached the pH between 6 and 7. Then, the carboxylated SWCNT residue was dried in the oven at 50 °C over night. After that, acid chloride functionalized SWCNTs were prepared by treating –COOH functionalized SWCNTs with thionyl chloride in the presence of DMF catalyst under argon medium [20–27]. Then the acid chloride group was used as chemical anchors for further derivatization with 4-aminothiophenol solution

(ATP) as shown in Figure 1. For this purpose, acyl-chlorinated SWSWCNTs were reacted with 4-aminothiophenol solution (ATP) (1  104 M) in 70% ethanol and 30% water and refluxed at 70– 80 °C for 3 h. The reaction mixture was then stirred for another 2 h while the solution mixture was cooling down. Subsequently the mixture was centrifuged at 7500 rpm for 20 min to remove unreacted ATP. As shown in Figure 1B, though SWCNT is not soluble in water, ATP attached SWCNTs are highly soluble in water. Now to understand whether ATP has been attached with SWCNT or not, we have performed FTIR spectra. As shown in Figure 1C, we have seen clearly all the characteristic peaks for ATP [29] and these are –SH and –CS stretching vibrations at 2524 and 1088 cm–1, respectively. The absorption peak at 1306 cm–1 is –CN stretching vibration. The 3448 cm–1 peak is an –NH asymmetrical stretching vibration and the one at 3359 cm–1 is the symmetrical stretching vibration of –NH. All the other peaks are mainly due to the SWCNT, –C@O and –C@C– stretching vibration bands of ATP [20–29]. Next, gold nanoparticle was attached with SWCNT through –SH linkage via ATP. For this purpose, 100 lL SWCNT functionalized ATP was added to 0.1 mL of popcorn or rod nanoparticle solution. The resulting solution was left undisturbed for 30 min. The green and violet color of rods and popcorn shape gold nanoparticles changed into darker color (as shown in Figures 2 and 3)1 indicating the formation of SWCNT/GNP, respectively. Figures 2 and 3 shows the TEM picture of rod shape gold nanoparticle conjugated SWCNT and popcorn shape gold nanoparticle conjugated SWCNT. Our data clearly show that the rod shape gold nanoparticles and popcorn shape gold nanoparticles are nicely decorated on SWCNTs. Figure 2C shows the absorption spectra of ATP modified SWCNT, gold nanorod (GNR) and GNR attached SWCNT hybrid. Broad and structureless absorption spectrum from near infra-visible regions from 400 to 850 nm is mainly due to the E11 and E22 transitions of nanotubes [20–28]. As shown in Figure 2C, in case of rod shape gold nanoparticle we observed a two long wavelength plasmon bands around 520 nm and 680 nm. 520 nm band is due to the coherent electronic oscillation along the short axis (transverse absorption band) and 680 nm band is due to the coherent electronic oscillation along the long axis (longitudinal band). Absorption maximum of the longitudinal band is sensitive to the rod length. When we attached rod shaped gold nanoparticle with SWCNT, the absorption spectrum seemed like from the mixture of both and very broad band between 680 and 900 nm, as shown in Figure 2C. This broad band is mainly due to the fact that in SWCNT/GNR hybrid, rod shape gold nanoparticles are in close contact. Figure 3C shows the absorption spectra of only SWCNT, popcorn shape gold nanoparticle (GNS) and GNS attached SWCNT. As shown in Figure 3C, in case of popcorn shape gold nanoparticle, we observed a strong long wavelength plasmon band around 550 nm and it is due to the oscillation of the conduction band electrons [8]. When we attached popcorn shape gold nanoparticle with SWCNT, the absorption spectrum for SWCNT/GNS hybrid seems like from the mixture of both, as shown in Figure 3C. Only change we observed is that the 550 nm band becomes very broad and it is mainly due to the fact that in SWCNT/GNS hybrid, popcorn shape gold nanoparticles are in close contact, as shown in Figure 3A. As we discussed, in gold nanoparticle hybrid SWCNT gold nanoparticles are in close contact and as a result, it increased the number of hot spots. Since hot spot sites have a large electromagnetic field, it provides a significant enhancement of the Raman signal intensity from SWCNTs by 1.5 orders of magnitude, as shown in

1 For interpretation of color in Figures 2 and 3, the reader is referred to the web version of this article.

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Figure 1. (A) Schematic representation shows the synthesis protocol for the formation of rod shape gold nanoparticle attached SWCNTs. (B1) Only SWCNT in water solution, (B2) ATP modified SWCNT in water solution. (C) FTIR spectra of ATP attached SWCNT.

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Figure 2. (A) TEM image showing rod shape gold nanoparticle functionalized SWCNTs. (B) Picture showing color of (B1) ATP attached SWCNT, (B2) only GNR and (B3) SWCNT/GNR hybrid. (C) Absorption spectra for rod shape gold nanoparticle, ATP modified SWCNT and rod shape gold nanoparticle attached SWCNT.

Figure 4. Our SERS spectrum shows two clear SWCNT characteristic Raman bands between 1000 to 1650 cm1 and these are: disorder

related mode (D band) around 1300 cm1 and tangential graphitelike mode (G band) around 1590 cm1 [20–28]. As shown in Fig-

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Figure 3. (A) TEM image showing popcorn shape gold nanoparticle functionalized SWCNTs. (B) Picture showing color of (B1) ATP attached SWCNT, (B2) SWCNT/GNS hybrid and (B3) only GNS. (C) Absorption spectra for popcorn shape gold nanoparticle, ATP modified SWCNT and popcorn shape gold nanoparticle attached SWCNT.

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Raman Shift (cm-1) Figure 4. Plot showing SERS enhancement in the presence of gold nanoparticle for GNR/SWCNT hybrid and GNS/SWCNT hybrid.

ure 4, Raman intensity of SWCNT bands increase by 1.5 orders of magnitude once SWCNT has been decorated with popcorn shape gold nanoparticle. Similarly, we have observed about one order of magnitude enhancement of SWCNT Raman bands in case of SWCNT/GNR hybrid. It is generally believed that SERS enhancement in the presence of metal nanoparticles are mainly due to two fundamentally different mechanisms [1–18] and these are an electromagnetic enhancement effect associated with large local E-field and a chemical enhancement effect due to the electronic interaction between the molecule and metal surface. The electromagnetic contribution is believed to be several orders of magnitude more than the value for the chemical enhancement [1–18]. When two nanoparticles are placed close to each other as in the case of SWCNT/GNS hybrid, the polarization direction of the E-field is along the axis of the two particles. As a result, the local E-field in the gap can be further enhanced under the resonance condition, forming ‘hot’ site for Raman scattering. This ‘hot spot’ formation helps to increase SERS intensity more than one order of magnitude for SWCNT/GNP hybrid. Our results also show that SERS enhancement for popcorn shape gold nanoparticle hybrid SWCNT is higher than rod shape gold nanoparticles hybrid SWCNT.

It is mainly due to the fact that in nano-popcorn, the central sphere acts as an electron reservoir while the tips are capable of focusing the field at their apexes which will provide sufficient field of enhancement [13]. As a result, the low cross-section Raman signals has been amplified several orders of magnitude particularly in narrow nanoscaled corners and edges. In the presence of sharp corner or edge, quasi-electrostatic crowding of many electric field lines leads to a tremendous field enhancement, which is known as lightning rod effect [10–18]. This is very important as long as the effective curvature of the sharp feature is much smaller than the wavelength of interest. Now, in our case it is obvious that the number of tips for popcorn shape gold nanoparticle is more and tips of the popcorn shape gold nanoparticles are much sharper than the ends of the rod shape gold nanoparticles. As a result, the local electric field enhancement associated with the corner or tips are much larger for popcorn shape gold nanoparticles than that of the rod shape gold nanoparticles. So our experimental result clearly shows that shape control can be used to take advantage of the ‘lightning rod effect’ in order to produce extraordinarily high SERS enhancement. To demonstrate that due to the presence of huge surface area, gold nanoparticle attached SWCNTs are useful for SERS substrate, we have added well characterized Rh-6G dyes at different concentrations in the SWCNT/GNS hybrid solution. After allowing 40 min for thermodynamic equilibrium, 20 lL aliquots were cast and airdried on glass slides (as shown in Figure 5) and their SERSenhancement properties were evaluated using 785 nm excitation laser light. As shown in Figure 5, very high (1012 orders of magnitude) SERS enhancement was observed and it is due to the broad plasmon band of the hybrid and the formation of a high density of hot spots on the coated SWCNT surface. As shown in Figure 5, in case 785 nm excitation, we see a very nice SERS spectra from 109 M Rh6G adsorbed on SWCNT/GNS hybrid, on the other hand SERS spectra from 108 M Rh6G adsorbed on only GNS is almost negligible. It is due to several factors and these are, (1) incident light is not in resonance with 108 M dye adsorbed GNS, whereas in case of hybrid nanomaterial, the incident light is in strong resonance, as shown in Figure 3. (2) The largest Raman scattering enhancements, even single molecule SERS, have been described for molecules residing in the fractal space between aggregated colloidal nanoparticles [1–18]. As we discussed before that in hybrid nanomaterial popcorn shape gold nanoparticle are in close contact and as a result, we have noted about twelve orders

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Excitation Wavelength (nm) Figure 5. (A) Photograph showing SWCNT/GNS film cast on glass slides. (B) Plot showing SERS enhancement of Raman signal at 785 nm excitation from Rh6G, in the presence of only popcorn shape gold nanoparticle and popcorn shape gold nanoparticle/SWCNT hybrid. (C) Plot showing wavelength dependent SERS enhancement factors for 1511 cm1 Raman band.

of magnitude enhancement of Raman signal (as shown in Figure 5) in case SWCNT/GNS hybrid thin film. The Raman modes at 615, 778, 1181, 1349 1366, 1511, 1570, 1603, and 1650 cm–1 are due to C–C ring in-plane bending, C–H out-of-plane bending, C–N stretching and C–C stretching, as we reported before [8–18]. In case of Rh6G adsorbed on SWCNT/GNS, we also observed strong G band around 1590 cm1.The Raman enhancement, G, is measured experimentally by direct comparison as shown below [8– 18],

G ¼ ½ISERS =½IRaman   ½Mbulk =½M ads  where ISERS is the intensity of a 1511 cm1 vibrational mode in the surface-enhanced spectrum in the presence of SWCNT/GNP hybrid nanomaterial, and IRaman is the intensity of the same mode in the bulk Raman spectrum from only Rh6G. Mbulk is the number of molecules used in the bulk, Mads is the number of molecules adsorbed and sampled on the SERS-active substrate. All spectra are normalized for integration time. An enhancement factor estimated from the SERS signal and normal Raman signal ratio for 1511 cm1 band is approximately 8.9  1011 in case of SWCNT/GNS hybrids at 785 nm excitation. No significant changes in Raman frequencies are observed in comparison to the corresponding SERS and Raman bands. It is known that when the molecule is excited at the plasmon resonance wavelength of the noble metal nanostructures, signifi-

cantly enhanced electromagnetic (EM) fields arise which is responsible for the major enhancement in SERS [1–18]. The EM enhancement can give rise to enhancement factors up to 108 [1– 18]. To understand this electromagnetic mechanism of SERS enhancement in our hybrid nanoparticle, we have monitored wavelength dependence of SERS intensity. For this purpose, we have used 632.8, 676, and 785 nm excitation. As shown in Figure 5C, at 785 nm excitation, the SERS enhancement factor for 1511 cm1 Raman band is about 1012 in case of SWCNT hybrid popcorn shape gold nanoparticle adsorbed Rh6G, whereas SERS enhancement factor for the same band is only around 105 in case of only popcorn shape gold nanoparticle adsorbed Rh6G. Since near IR 785 nm light is very good for biological window, our data show that hybrid nanomaterial based SERS substrate will be very useful for biological assay applications. Our result shows that the wavelength dependent SERS enhancement factor is highly dependent on the resonance condition of the excitation wavelength. 3. Conclusions In conclusion, in this Letter, we have reported chemical design of gold nanoparticle of different shapes decorated with SWCNT based ultrasensitive SERS substrate. We have shown that since in hybrid material, gold nanoparticles are in close contact, they form several hot spots and provide a significant enhancement of the Ra-

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man signal intensity of SWCNTs D and G band through electromagnetic field enhancements. Our shape dependent result shows that popcorn shape gold nanoparticle decorated SWCNTs will be the best choice for SERS probe due to the existence of several sharp edges or corners. We have demonstrated that SWCNT/GNS based SERS substrate is capable of providing SERS enhancement of about twelve orders of magnitude. Our results not only provide a good approach to ultrasensitive SERS substrates, but are also helpful for improving the fundamental understanding of SERS phenomena. Despite very good SERS performances, we need to admit that our SERS substrates need to be further optimized by tuning nanoparticle size, interparticle gap and SWCNT alignment. After optimization of these parameters, we believe that this hybrid nanotechnology driven SERS substrate could have enormous potential applications. Acknowledgments Dr. Ray thanks NSF-PREM grant # DMR-0611539, NSF-CREST grant # HRD-0833178 and DOD grant # W 912HZ-06-C-0057 for their generous funding. We also thank reviewers whose valuable suggestions have improved the quality of the manuscript. References [1] [2] [3] [4]

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