Bifunctional Nanocomposites with Long-Term Stability as SERS

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2009, 113, 3373–3377 Published on Web 12/22/2008

Bifunctional Nanocomposites with Long-Term Stability as SERS Optical Accumulators for Ultrasensitive Analysis Miguel Spuch-Calvar,† Laura Rodríguez-Lorenzo,† M. Puerto Morales,‡ ´ lvarez-Puebla,*,† and Luis M. Liz-Marza´n*,† Ramo´n A. A Departamento de Química Física and Unidad Asociada CSIC, UniVersidade de Vigo, 36310 Vigo, Spain, and Instituto de Ciencia de Materiales de Madrid, CSIC, 28049 Cantoblanco, Madrid, Spain ReceiVed: NoVember 28, 2008

In this work, we have devised and fabricated a magnetic + optical, bifunctional colloidal system that combines flexible handling and efficient SERS analytical capabilities. This system comprises silica-coated magnetic γ-Fe2O3 (maghemite) cores, coated with a dense monolayer of gold nanorods presenting long-term optical stability and a high density of hot spots per area unit. The magnetic functionality allows for the use of a small number of capsules that can be later concentrated under a magnetic field for SERS analysis thereby increasing the detection limits. Introduction The recent progress in the morphology-controlled synthesis and the assembly of metal nanoparticles, as well as in the understanding of their optical response, have contributed to the advancement of various research fields and applications, such as photocatalysis,1 nanophotonics2 or biomedical imaging,3 diagnostics4 and therapy.5 Among these applications, surfaceenhanced Raman scattering (SERS)6 has been identified as a powerful analytical technique with a bright potential for ultrasensitive chemical or biochemical analysis.7 The principle behind SERS is an enormous enhancement of Raman scattering cross section when the molecules are close to a metallic surface, which originates mainly from the large electric fields associated with surface plasmon resonances.8 For practical applications, it is necessary to devise suitable substrates, with long-term stability and large enhancement efficiency. The most important SERS substrates are currently colloidal nanocrystal suspensions and ultrathin films of gold, silver, and their alloys, produced by a variety of physical and chemical methods.9 Metal colloids are advantageous because their large surface area and their dispersion in liquids allows for a close adsorbent-adsorbate interaction, so that the analyte can be naturally retained onto the nanoparticle’s surface,10 and subsequently studied, either directly by “average SERS” or cast onto appropriate substrates (glass or silicon wafers) and analyzed by “hot-spot SERS”.6 However, the colloidal stability of these dispersions is typically compromised because efficient SERS analysis requires clean surfaces (free of strongly adsorbed organic moieties), and thus the shape, size, or surface properties can change within days of preparation. Moreover, the molar ratio between colloidal suspension and analyte solution should be very high for an effective SERS experiment, in the order of 1 mL of colloids (∼10-3-10-4 M in metal) per 10 µL of analyte suspension (∼10-3-10-9 M). * To whom correspondence should be addressed. E-mail: ramon.alvarez@ uvigo.es; [email protected]. † Universidade de Vigo. ‡ CSIC.

10.1021/jp810470n CCC: $40.75

Such a high colloid nanoparticles:analyte molecules ratio is necessary to probe a large number of particle enhancers per unit volume in average SERS or to achieve a sufficient amount of hot spots when the suspension is cast. This large amount of colloidal particles decreases the detection limits of SERS because all of the particles adsorb the same amount of analyte molecules but only a few of them are actually analyzed in the sample. Thus, reducing the number of adsorbing particles would permit complete saturation at low analyte concentration, with an increase in detection limit if a large amount of the particles can be collected for the SERS measurement. The alternative substrates for SERS are metal thin films, which can be implemented in the form of portable substrates with strong optical-enhancement properties due to the existence of hot spots, but are less flexible because average measurements cannot be carried out, they are single-use platforms, and the fabrication methods are not commonly available in spectroscopy laboratories. In this paper, we present a composite, bifunctional colloidal system that combines flexible handling and efficient SERS analytical capabilities. This system comprises silica-coated magnetic γ-Fe2O3 (maghemite) cores, coated with a dense monolayer of gold nanorods. Such composite particles present long-term optical stability, and additionally combine in a single platform the main advantages of colloidal and film SERS substrates: (1) the colloidal support provides long-term colloidal stability, allowing for extensive cleaning of the rod surfaces; (2) the rods are in close contact on the particles surface while in suspension, thus generating stable hot spots that yield high SERS intensities; (3) the magnetic functionality allows one to use a small number of particles that can be later concentrated under a magnetic field for SERS analysis; and (4) the longterm colloidal and optical stability permits the storage of the material for extended periods of time in, for example, eye droppers, without change in either the optical or the magnetic properties, allowing for their use whenever necessary.  2009 American Chemical Society

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Experimental Section Synthesis of Maghemite Spindles. γ-Fe2O3 (maghemite) spindles were prepared by H2 reduction at 360 °C and oxidation at 240 °C of R-Fe2O3 (hematite) particles with the same morphology11 prepared by homogeneous hydrolysis at 100 °C of FeCl3 (0.02 M) in the presence of KH2PO4 (4.5 × 10-4 M).12 The average length was 540 nm ((70), with an average width of 90 nm ((10). Silica coating was obtained following a previously reported method.13 Iron oxide particles were dispersed in a solution containing 2-propanol (100 mL) and ammonia solution (5 mL, 8 M). Tetraethyl orthosilicate (TEOS, 0.01 mol) was then added and the resulting dispersion stored in a tightly closed container at 40 °C for 16 h under stirring. The average thickness of the obtained silica shell was 75 nm. Polyelectrolyte Coating. Surface modification of the silicacoated γ-Fe2O3 spindles was carried out by layer-by-layer selfassembly14 using positively charged poly diallyldimethylammonium chloride (PDDA) and negatively charged polystyrene sulfonate (PSS). Briefly, PSS (1 mL, 1 mg/mL in NaCl 0.5 M) was added under magnetic stirring to 1 mL of the washed γ-Fe2O3 colloids in water (1.6 g L-1). After a few minutes, excess PSS was removed by centrifugation and washing with Milli-Q water. The process was repeated with PDDA (1 mg/ mL in NaCl 0.5 M) and again with PSS and PDDA, until obtaining an external, uniform positive surface. The efficiency of the various coating steps was monitored through zeta potential measurements, obtaining values with alternately negative and positive charge (-47.1, +47.2, -41.1,..., +47.1 mV). Synthesis and Assembly of Gold Nanorods. Gold nanorods were prepared following the seed mediated method.15 Briefly, Au seeds were formed by adding NaBH4 (0.6 mL, 0.01 M) to HAuCl4 (5 mL, 0.5 mM), in the presence of CTAB 0.1 M under stirring. The growth solution was prepared by adding in this order: HAuCl4 (5 mL, 0.5 mM), CTAB (5 mL, 0.2 M), AgNO3 (0.08 mL, 0.004 M), ascorbic acid (0.1 mL, 0.788 M), and seed solution (0.09 mL). The rods were functionalized with PVP by centrifuging, washing in a small volume of water, and adding a polymer concentration 1.2 fold larger than the molar concentration of gold. Finally, the solution was centrifuged and transferred into ethanol. Assembly of the nanorods was achieved through electrostatic interactions between the negative PVP layer on the nanorods and positively charged PDDA on silica coated maghemite. Briefly, γ-Fe2O3 (5 mL, 1.5 g L-1) was added dropwise to the rod suspension (250 mL, 0.44 mM) under stirring and sonication. The blend was stirred during one day and extensively washed. Characterization. Optical characterization was carried out by UV-vis-NIR spectroscopy with a Cary 5000 spectrophotometer, using 10 mm path length quartz cuvettes. Transmission electron microscopy images were obtained with a JEOL JEM 1010 transmission electron microscope (TEM) operating at an acceleration voltage of 100 kV, while scanning electron microscopy (SEM) images were obtained with a JEOL JSM6700F field-emission scanning electron microscope (FE-SEM) operating at 5 kV in secondary electrons image (SEI). Magnetic characterization was carried out in a vibrating sample magnetometer (MLVSM9 MagLab 9 T, Oxford Instrument). Magnetization curves were recorded at room temperature by first saturating the sample in a field of 3 T; the saturation magnetization (Ms) values were evaluated by extrapolating to infinite field the experimental results obtained in the high field range where the magnetization linearly increases with 1/H. Average SERS. Samples for average SERS on free rods and γ-Fe2O3@rods were prepared as follows: suspensions were

Figure 1. Schematic drawing, SEM, and TEM images of gold nanorods and γ-Fe2O3@rod composite particles.

repeatedly washed by centrifugation and redispension in water to gradually remove adsorbed PVP from the metallic surfaces. Then 1-naphthalenethiol (1NAT) (10 µL, 10-3 M) was added to 1 mL of either rods or γ-Fe2O3@rods (both 0.15 M in gold), after each washing cycle. After 2 h, enough for thermodynamic equilibrium to be reached, SERS spectra were measured with a LabRam HR system (Horiba-Jobin Yvon), equipped with a confocal optical microscope, high resolution gratings (1800 g mm-1) and a Peltier CCD detector by using a macro-sampler accesory. For the sample excitation, a near-infrared laser line (785 nm) was used with acquisition times of 10 s and power density at the sample of 7 mW. Ultrasensitive SERS Analysis. Samples for ultrasensitive analysis were prepared as follows. (1) γ-Fe2O3@rods samples: γ-Fe2O3@rods (50 µL, [Au] ) 0.15 mM) was added to analyte solutions (10 mL) with concentrations ranging from 10-5 to 10-11 M. The final concentration of composite particles in the suspension was adjusted to 7.5 × 10-7 M in Au. Samples were stirred for 2 h and then, magnetic particles were collected at the bottom of the vial with a hand-held magnet (110 mT). After magnetic concentration, the supernatant was removed and a 10 µL aliquot of the concentrated residue was cast on a glass slide. Cast samples were studied with a LabRam HR system (HoribaJobin Yvon). The excitation laser beam (785 nm) was focused onto the sample using a 100 × objective (NA 0.95), providing spatial resolution of about 800 nm2. Power density at the sample was 1 mW with acquisition times of 10 s. (2) Free rod sample: 10 µL of the analyte solution was added to the rod dispersion (1 mL, [Au] ) 0.15 mM). The final concentration of analyte in the samples ranged from 10-5 to 10-11 M. After 2 h, the samples were centrifuged and 10 µL of the concentrated residue was cast on a glass slide. Samples were studied as described above. SERS Measurements in a Portable System. SERS vials were prepared by dispersing γ-Fe2O3@rods (50 µL, [Au] ) 0.15 mM) in water (1 mL). The analyte solution (10 µL) was added to the suspension and allowed to equilibrate for 2 h. Then the composite particles were concentrated with a hand-held magnet at a focalized spot on the inner wall of the vial. Raman spectra were acquired in situ with a portable Raman system (Inspector Raman Hand-Held Field Analysis Spectrometer, Delta Nu) equipped with 785 nm excitation source and a single grating. Results and Discussion Figure 1 shows a schematic drawing of the composite colloidal particles, as well as representative TEM and SEM images of the free gold nanorods and the same rods supported onto the silica-coated maghemite spindles (γ-Fe2O3@rod). Details on the synthesis and assembly are provided in the

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Figure 3. (a) Average SERS spectrum of 1-naphthalenethiol and twodimensional plots showing the variation of the SERS spectra along various washing steps, on free rods and γ-Fe2O3@rod, as labeled. (b) Variation of the intensity of 1368 cm-1 band (ring stretching) of 1NAT, with washing. See Experimental Section for details.

Figure 2. Top: UV-vis spectra of free gold nanorods, maghemite spindles, maghemite spindles coated with rods, and the composites with the support subtracted; the yellow dotted arrow indicates the excitation laser line (785 nm). Bottom: Room temperature magnetization curves for naked maghemite and the γ-Fe2O3@rod composites.

Experimental section. The core-shell structure of the magnetic supports is clearly demonstrated by the TEM micrographs, where the silica shell is distinguished as a lighter area (ca. 75 nm thick) surrounding the maghemite spindle core. The gold nanorods can also be observed as darker spots on the silica surface. The average dimensions of the nanorods are 33 ( 5 × 13 ( 2 nm, with an average aspect ratio of 2.7 ( 0.6. The longitudinal plasmon resonance of free nanorods is centered at 700 nm (Figure 2), red-shifting to 775 nm upon assembly onto the silica-coated maghemite particles, which actually becomes 790 nm upon subtraction of the extinction by the γ-Fe2O3 support. This red-shift is attributed to plasmon coupling between neighboring nanorods16 and may be considered as an evidence of the generation of hot spots on the capsule surface.17 Moreover, and in close agreement with the reported surface-enhanced Raman excitation spectroscopy,18 the perfect match of the LSPR with the excitation laser line, 785 nm in this case, was expected to provide a larger SERS enhancement. Thus, this bifunctional hybrid material combines optimal morphology, high density of hot spots and perfect LSPR-excitation laser line coupling, rendering it an excellent support for NIR-SERS. It should be however noted that, while this is a first demonstration of the system, the optical properties of the enhancing nanoparticles may be as well tailored for efficient excitation with either visible or IR laser lines.19 The magnetic properties of these composite particles are extremely interesting, since the maghemite spindles display a much higher magnetization at low fields, as compared to similar systems previously reported, where either complete gold shells20 or gold nanorods21 were deposited on hematite (RFe2O3). The room temperature hysteresis loops measured from powder samples of both the (uncoated) γ-Fe2O3 spindles and the composite particles are shown in Figure 2, bottom. The measured saturation magnetization values are 74 emu/g for γ-Fe2O3 and 10.8 emu/g for γ-Fe2O3@rod particles (where the

total weight of the composites was considered, including the silica shell and the gold rods). The coercivity value (Hc ) 250 Oe) at room temperature shows an increase of 25% for γ-Fe2O3@rod, as compared to γ-Fe2O3 (Hc ) 200 Oe), due to a reduction of interparticle interactions, as previously reported for silica coated maghemite.13 The SERS properties were initially tested in solution by means of average SERS. Both free rods and rod-maghemite composite colloids were washed several times (by centrifugation and redispersion in milli-Q water) and SERS spectra were collected after each washing cycle, upon addition of 1-napthalenethiol (1NAT) as molecular probe, with a final concentration of 10-5 M. Figure 3a shows the SERS spectrum of 1NAT, which is dominated by the ring stretchings (1553, 1503 and 1368 cm-1), CH bending (1197 cm-1), ring breathings (968, and 822 cm-1), ring deformations (792, 664, 539, and 517 cm-1), and CS stretching (389 cm-1).22 Notably, for both rods and γ-Fe2O3@rod, the SERS signal increased for cleaner metallic surfaces after the first washing cycle (Figure 3a,b). However, while the SERS signal from γ-Fe2O3@rod continued to increase up to the fourth washing cycle (remaining constant thereafter), the SERS signal from the rods colloid only increased after the first two cycles, but dramatically decreased after the third cycle. This drop of the SERS signal can be interpreted as a consequence of the loss of colloidal stability as the protective polymer, polyvinylpyrrolidone (PVP) in this case, was removed, clearly demonstrating the enhanced colloidal stability provided by the solid-phase support in the composites. It is also clear from Figure 3 that, during the entire washing process the composites promote a considerably higher SERS intensity, in the order of 5.5 fold difference when the maximum SERS signal from the composites (5th washing cycle, Figure 3b) is compared to the corresponding maximum from the rods (2nd washing cycle, Figure 3b). Such an intensity difference is mainly ascribed to the formation of hot spots on the composite surface,23 as well as a better coupling between LSPR and excitation laser line.24 The detection limits that can be obtained from pure rods and bifunctional composites were studied from measurements on films that were cast on glass slides. For preparation of the films, the rod dispersion (0.15 M in gold) was centrifuged and a 10 µL aliquot of the concentrated residue was cast on the glass slide. On the other hand, a composite colloid with a gold

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Figure 4. Variation of the SERS intensity as a function of analyte concentration (p[1NAT] ) -log[1NAT]) for (a) γ-Fe2O3@rod and (b) free rods. (c) SERS spectra corresponding to 1NAT concentrations of 10-10 and 10-11 M on composite colloids, and 10-8 and 10-9 M on free rods. (d) White light optical image (top) and SERS mapping (bottom) of capsules doped with 1NAT (5 × 10-10 M).

concentration of 7.5 × 10-7 M (50 µL of capsule suspension, with a gold concentration 0.15 M, redispersed into 10 mL of analyte solution) were concentrated by collecting the particles with a magnet at the bottom of the vial (see Experimental Section for details), and casting a 10 µL aliquot of the concentrated residue on the slide. Figure 4a,b confirms that the SERS intensity decreased for lower analyte concentrations. Interestingly, pure rod samples showed higher intensity at larger analyte concentrations, but as 1NAT concentration was decreased, the enhancement provided by the composites became significantly higher, which can be explained on the basis of the respective sample preparation. In the case of γ-Fe2O3@rod composite particles, the total concentration of gold was 200 fold lower than that of pure nanorods. Thus, when the analyte concentration is sufficiently high all metal nanoparticles can adsorb 1NAT molecules and therefore, since the density of rods on the substrate is much larger than that of rods on the composite particles, the obtained signal is accordingly more intense (It should be noted that casted rods form a large density of hot spots). However, as 1NAT concentration is decreased, the nanoparticle:analyte ratio increases; with a considerably larger effect on the intensity from the system containing a higher metal particle concentration. Therefore, the intensity provided by pure rods rapidly decays while the signal from the composite sample is still strong down to 1NAT concentrations of 10-7 M. Moreover, the signal from the cast rod system is completely lost at [1NAT] ) 10-9 M while, in the case of the composites, the SERS spectrum is still clearly identified at [1NAT] ) 10-10 M (Figure 4c). Calculation of the enhancement factors for free rods and γ-Fe2O3@rod provides a value of 144 fold increase for the latter. It is worth stressing the following points: (1) the use of free rods requires higher particle concentrations to achieve sufficient intensity, and concentration of very dilute solutions by centrifugation is not possible; (2) Fe2O3@rod capsules behave as an optical signal accumulator as they can be saturated with the analyte at extremely low particle concentrations (i.e., smaller amounts of adsorbent solid will reach saturation at smaller analyte concentrations) because magnetic separation can be achieved; and, (3) SERS mapping (Figure 4d) demonstrated that a very small amount of capsules is sufficient to obtain a clear

signal for identification of the analyte, further illustrating the high activity of the hot spots generated at their surfaces. A final advantage arising from the bifunctional character of the composite particles is their potential to be used as portable substrates. The idea consists of using sample vials containing minute amounts of γ-Fe2O3@rod particles (for example, 50 µL of a 1.54 g L-1 dispersion diluted in 1 mL of water; i.e., equivalent to 1.48 mg of Au per liter of solution), so that the liquid containing the analyte of interest can be added and, upon thermodynamic equilibrium, the composite particles can be collected on a focalized spot of the vial by means of a handheld magnet (Figure 5a). Samples prepared in this way can then be measured, either in situ by using a commercially available, portable Raman system or stored for further analysis. As a demonstration of this capability for in situ ultradetection, Figure 5b shows SERS spectra of 1NAT acquired using a portable Raman spectrometer. The detection limit in this experiment was of the order of 10-7 M, which was likely due to scattering interference from the glass and lower performance of the detector in the portable system.

Figure 5. (a) Photograph showing the 2 vials containing the analyte solution with minute amount of γ-Fe2O3@rod capusles (50 µL) dispersed and concentrated at the wall by applying a punctual magnetic field with a conventional magnet. (b) SERS spectra of 1NAT at different concentrations acquired on the concentrated spot of the wall by using a portable Raman system. See Experimental Section for details.

Letters Conclusions In conclusion, we have devised and fabricated a magnetic+ optical, bifunctional accumulator for SERS that offers unique advantages over existing substrates. It combines the virtues of both colloids and films, allowing measurements both in suspension and supported on slides. γ-Fe2O3@rod capsules offer a clean substrate for SERS application with a high density of hot spots and the possibility of nanoparticle optimization for each specific application or excitation laser line. The magnetic functionality allows for the fast separation of minute amounts of material. Thus, very small quantities of colloid particles are actually required for effective SERS analysis, consequently lowering the detection limits as compared with free nanorods, in over 2 orders of magnitude. Additionally, these systems are stable over long periods of time, both regarding their optical and magnetic response, as well as the colloidal stability. Therefore, concentrated colloids can be stored and used in small volumes (∼50 µL) for conventional SERS applications, potentially carried out in portable Raman systems. Acknowledgment. We thank B. Rodríguez-González (CACTI, U. Vigo) for assistance with SEM measurements. This work was funded by the Spanish Ministerio de Educacio´n y Ciencia, under Contracts NAN2004-08843 and MAT2007-62696, and by the Xunta de Galicia. References and Notes (1) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943–4950. Hirakawa, T.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 3928–3934. (2) Lal, S.; Link, S.; Halas, N. J. Nat. Photonics 2007, 1, 641–648. Curto, A. G.; García de Abajo, F. J. Nano Lett. 2008, 8, 2479–2484. Bek, A.; Jansen, R.; Ringler, M.; Mayilo, S.; Klar, T. A.; Feldmann, J. Nano Lett. 2008, 8, 485–490. (3) Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. J. Am. Chem. Soc. 2006, 128, 2115–2120. Qian, X.; Peng, X. H.; Ansari, D. O.; YinGoen, Q.; Chen, G. Z.; Shin, D. M.; Yang, L.; Young, A. N.; Wang, M. D.; Nie, S. Nat. Biotechnol. 2007, 26, 83–90. (4) Shah, N. C.; Lyandres, O.; Walsh, J. T., Jr.; Glucksberg, M. R.; Van Duyne, R. P. Anal. Chem. 2007, 79, 6927–6932. Kneipp, J.; Kneipp, H.; Wittig, B.; Kneipp, K. Nano Lett. 2007, 7, 2819–2823. (5) Dickerson, E. B.; Dreaden, E. C.; Huang, X.; El-Sayed, I. H.; Chu, H.; Pushpanketh, S.; McDonald, J. F.; El-Sayed, M. A. Cancer Lett. 2008, 269, 57–66. Choi, M.-R.; Stanton-Maxey, K. J.; Stanley, J. K.; Levin, C. S.; Bardhan, R.; Akin, D.; Badve, S.; Sturgis, J.; Robinson, J. P.; Bashir, R.; Halas, N. J.; Clare, S. E. Nano Lett. 2007, 7, 3759–3765. (6) Aroca, R. F. Surface enhanced Vibrational spectroscopy; Wiley: New York, 2006. (7) Bonham, A. J.; Braun, G.; Pavel, I.; Moskovits, M.; Reich, N. O. J. Am. Chem. Soc. 2007, 129, 14572–14573. Barhoumi, A.; Zhang, D.; Tam, F.; Halas, N. J. J. Am. Chem. Soc. 2008, 130, 5523–5529. Feng, M.; Tachikawa, H. J. Am. Chem. Soc. 2008, 130, 7443–7448. Qian, X.-M.; Nie,

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