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Achieving high-purity colloidal gold nanoprisms and their application as biosensing platforms
Journal of Colloid and Interface Science 348 (2010) 29–36
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Achieving high-purity colloidal gold nanoprisms and their application as biosensing platforms Zhirui Guo a, Xu Fan b, Lianke Liu a, Zhiping Bian a, Chunrong Gu a, Yu Zhang b, Ning Gu b, Di Yang a,*, Jinan Zhang a,** a b
Institute of Cardiovascular Disease, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China State Key Laboratory of Bioelectronics and Jiangsu Laboratory for Biomaterials and Devices, Southeast University, Nanjing 210096, China
a r t i c l e
i n f o
Article history: Received 9 December 2009 Accepted 9 April 2010 Available online 18 May 2010 Keywords: Gold Nanostructure Electrostatic interaction Anisotropic shape Biosensor
a b s t r a c t Gold nanoprisms with average edge size of 140 nm and thickness of 8 nm were achieved in highpurity (97%) by exploiting the electrostatic aggregation and shape effects through a modified seedmediated approach. The proposed strategy lies in the dramatically different stability and aggregation potential between the produced gold nanoprisms and spherical gold nanoparticles, which can be modulated by varying the anion concentration in the reaction solution. Hence, the gold nanoprisms spontaneously aggregated into precipitate whereas most of the spherical ones were still kept in the solution. Moreover, this strategy is also flexible enough that ultra-small gold nanoprisms with average width less than 50 nm can be collected in good-purity. The structure and optical properties of these nanoprisms have been studied by TEM, SAED, XRD and UV–vis–NIR spectroscopy, respectively. These high-purity colloidal gold nanoprisms exhibit remarkably enhanced surface plasmon resonance (SPR) as well as strong near-infrared absorption. Furthermore, we have also investigated their potential for biosensing based on the sensitive changes of SPR band induced by the antibody–antigen recognition events. The experimental results clearly suggest that gold nanoprisms can be a promising nanostructured system for plasmonic sensor applications. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Finely controlling the shape and size of metal nanostructures has become an active area of research because the optical, electrical, magnetic and catalytic properties of metal nanostructures are strongly dependent on their shape and size [1,2]. Thanks to the efforts of the worldwide research groups, multiplex shaped metal nanostructures, including icosahedras [3], decahedrons [4], octahedrons [5], bipyramids [6], cubes [7], rods [8], wires [9], belts (ribbons) [10], and other complex shapes such as multipods [11] or stars [12], have been synthesized during the last decade. In recent few years, gold and silver nanoprisms (nanoplates) have attracted great interest because of their unique optical properties known as localized surface plasmon resonance (SPR) and promising applications in photoluminescence [13], surface-enhanced Raman scattering [14], optical coating [15] and near-infrared (NIR) light absorbing [16]. In general, noble metal nanoprisms are a class of two-dimensional structures with parallel top and bottom and a nanoscale thickness. Although micrometer-sized gold or silver
* Corresponding author. Fax: +86 25 83738572. ** Corresponding author. Fax: +86 25 83738572. E-mail addresses: [email protected] (D. Yang), (J. Zhang).
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0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.04.013
prisms with thickness below 100 nm are commonly categorized as nanoprisms, they have not presented the unique optical properties associated with their colloidal analogs [17]. Furthermore, the solution of micrometer-sized prisms is prone to precipitate due to their large size, limiting the applications in solution phase. Up to date, a variety of synthetic strategies, such as photochemical, thermal and biological routes, have been explored to prepare bulk quantities of colloidal silver nanoprisms [18]. However, most of the synthesized gold nanoprisms have edge lengths ranging from submicrometer to tens of micrometer [19]. A few reports concerned with small gold nanoprisms often generate large amounts of other shaped structures difficult to high-efficiently separate [20]. Because gold is much more air-stable and biocompatible than silver, the achievement of colloidal gold nanoprisms with high-purity are quite desirable for expanding the application field which is presently unattainable by silver nanoprisms. Recently, Ha et al. reported the synthesis of small gold nanoprisms by a seed-mediated, cetyltrimethylammonium bromide (CTABr)-assisted approach [21]. In their work, a proper amount of iodide ion (I) was introduced into the reaction solution to repress the crystal growth along the h1 1 1i direction of gold, resulting in basal {1 1 1} facets bounded triangular gold nanoprisms. However, the final reaction mixture is still accompanied with a large fraction (55%) of spherical nanoparticles. In this paper we
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describe our recent research on the stability and aggregation potential between the gold nanoprisms and the spherical gold nanoparticles produced by the above-mentioned seeding synthetic system. It is found that colloidal gold nanoprisms with an excellent purity (97% in number density) can be achieved from the in situ reaction mixtures by exploiting the electrostatic aggregation effect, without conventional centrifugation or filtration [21,22]. The presented strategy is effective while flexible enough that ultra-small gold nanoprisms with average edge length less than 50 nm can be obtained in good-purity. Moreover, their SPR properties is studied and the potential as biosensing platforms using biomarker human cardiac troponin I as a protein sensing model is investigated. The experimental facts clearly demonstrate that gold nanoprisms can be a promising candidate for plasmonic sensor applications. 2. Materials and methods 2.1. Materials Cetyltrimethylammonium bromide (CTABr)1 and Poly(styrenesulphonate) (PSS, Mw 70,000) were purchased from Sigma. Tris(hydroxymethylamino)methane was received from Amresco. HAuCl4, sodium citrate, NaBH4, L-ascorbic acid (AA), NaOH, KI, K2CO3 and NaCl were all purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd. (China). All reagents were used as received without further purification. Millipore-quality water (18.2 MX cm) was used throughout the experiments. Tris Buffered Saline (TBS buffer) was prepared by dissolving 2.42 g tris(hydroxymethylamino)methane and 8.8 g NaCl in 900 mL of water. The solution was titrated with 1.00 M HCl until a constant pH value of 8.2 was obtained, and then water was added to receive a 1000 mL stock solution. Human cardiac troponin I (cTnI) from human myocardium muscle and the anti-human cTnI antibodies used in the experiment were obtained from Research Institute of Cardiovascular Disease of First Affiliated Hospital of Nanjing Medical University. All the glassware was cleaned by aqua regia (HCl:HNO3 in a 3:1 ratio by volume) and rinsed with water prior to the experiments. 2.2. Synthesis of gold seeds Gold seeds were synthesized by reducing 0.4 mL of 25 mM HAuCl4 with 1 mL of ice-cold 0.1 M NaBH4 with vigorously stirring. The reduction was done in the presence of 1 mL of 10 mM sodium citrate and 37.6 mL of water. Upon addition of the NaBH4, the solution turned a reddish orange color, indicating the generation of gold nanoparticles. The resulting solution was continually stirred for 2 min. The seed solution was allowed to stand for at least 2 h to ensure the complete hydrolysis of unreacted NaBH4. The gold seeds exhibited a SPR peak at 507 nm, and had an average diameter of less than 5.0 nm (see Supporting information). 2.3. Growth and achievement of gold nanoprisms with high-purity An initial reaction mixture of gold nanoprisms was prepared by a one-step seed-mediated, iodide ion- and CTABr-assisted approach with new modifications [21,22]. Typically, a 100 mL of growth solution containing 0.25 mM HAuCl4 and 0.05 M CTABr was prepared in a 150 mL beaker. To this solution was added 55 ll of 0.1 M KI, 1 Researchers have recently reported that the iodide ion impurity concentrations in CTABr vary from different manufacturers and the range is from less than 2.75 ppm to 840 ppm [44]. In current work, the needing [I] is greater than or equal to 50 lM. Thus to eliminate the side effects by the iodide ion impurity in CTABr, we have chosen Cat No: H6269 from Sigma, which contains the least iodide concentration (0.37 lM in our reaction solution) among the CTABrs.
0.55 mL of 0.1 M AA and 0.55 mL of 0.1 M NaOH (NaOH is used to deprotonate ascorbic acid for more effective reducibility) in turn and the resulting solution as growth solution was stirred gently. The orange color of the gold salt in the CTABr solution disappeared when AA was added, due to the reduction of Au3+ to Au+. The growth of gold nanoprisms was initiated by adding 0.1 mL of the seed solution to the growth solution. After the addition, the color of the growth solution changed from clear to light red, and then turned to deep claret-red over a period of 30 min. The reaction solution was kept at 30 °C and left undisturbed during the whole aging period. There were two ways for achieving high-purity colloidal gold nanoprisms: when the concentration of CTABr was greater than or equal to 0.05 M, the reaction mixture was aged without disturbance for 24 h; when the concentration of CTABr was lesser than 0.05 M, the reaction mixture was aged for 12 h, followed by adding NaCl until the sum of [Cl] and [Br] reached 0.2 M. After this, the reaction mixture was aged for another 12 h. Most of gold nanoprisms deposited on the bottom of the beaker during the aging period, and were easily collected by pouring out the supernatant. These gold products were redispersed to the colloidal solution for further characterization. 2.4. Conjugation of gold nanoprisms to anti-cTnI antibodies The high-purity nanoprisms in the CTABr solution was centrifuged to get rid of the extra free CTABr molecules. The nanoprism surface was coated with PSS polyelectrolyte before antibody conjugation. This was done by adding 2.4 ml of 2.5 mg/mL PSS to 30 mL nanoprism solution with an optical density of 1.0 at the long wavelength absorption maximum. The reaction was continued for about 15 min to complete the coating process. The extra PSS in solution was separated by centrifuging the prism solution at 3000 rpm. The pellet was redispersed in water and titrated with 0.2 M K2CO3 to reach a pH value of 8.2. Then the PSS capped nanoprism solution was mixed with an excess amount of anti-cTnI antibody (50 lg/mL) to react for 30 min. The nanoprisms conjugated with the anti-cTnI antibodies were centrifuged twice and redispersed into TBS buffer to form a stock solution with an optical density of around 1.0 at the long wavelength absorption maximum (washing buffer: 1% bovine serum albumin in TBS buffer to preclude nonspecific binding). 2.5. Binding of cTnI to anti-cTnI antibody conjugated gold nanoprisms In each experiment, 1.0 mL of anti-cTnI antibody conjugated gold nanoprisms was added to a fixed amount (10, 50, 100, 300, 500, 700 and 1000 ng) of cTnI at room temperature under vortex mixing. The resultant mixture was incubated for 15 min before recording with 2802S (UNICO) spectrophotometer. 2.6. Characterization The morphologies of the gold products were characterized by TEM (JEM-2000EX, JEOL) operating at 120 kV. The high-resolution TEM (HRTEM) image and the selected area electron diffraction (SAED) pattern were recorded on a JEOL JEM-2100 transmission electron microscope operating at 200 kV. The XRD analysis of the gold samples was carried out by using an X-ray diffractometer (D/Max-RA, Rigaku) with Cu Ka radiation at room temperature. The sample was deposited onto a silicon wafer and scanned in the 2h range of 30–90°. The Fourier transform infrared (FTIR) spectra were measured on a Magna FTIR-750 (Nicolet) spectrometer and the vacuum-dried samples were made in the form of KBr pellet. The UV–vis–NIR absorption spectra of the gold samples were collected by a UNICO 2802S spectrophotometer in a wavelength range from 300 to 1100 nm.
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3. Results and discussion The key point of our strategy lies in the different stability and aggregation potential between spheres and prisms. In a typical experiment conducted in the presence of 0.05 M CTABr, the growth of gold nanostructures has been monitored by UV–vis–NIR spectroscopy in real time. As shown in Fig. 1, two distinct SPR bands emerge within 2 min after seed addition. The first one appears at 532 nm and its intensity increases with time, which indicates that a large number of spherical gold nanoparticles are produced accompanying the growth of nanoprisms. The second band initially appears at 930 nm and this band red-shifts with the increase of the intensity as the growth continues, which indicates the presence of gold nanoprisms and the band shifts to NIR region could mirror an increase in edge length [23]. After 10 min, the intensity of the SPR band assigned to nanoprisms reaches its maximum and, unexpectly, begins to decrease gradually. At the meantime, the SPR band of spherical ones still keeps increasing in intensity. At the end of the aging period, the SPR band of gold nanoprisms disappears completely and only the SPR band of spherical ones is left, which reveals that the generated gold nanoprisms are much less stable than spherical ones and most of them aggregate spontaneously into precipitation. This precipitation can be collected easily by pouring out the supernatant. In particular, these as-obtained gold nanoprisms are prone to form colloidal solution by redispersion in water. As shown in Fig. 2a and b, both the spherical nanoparticles in supernatant and the triangular nanoprisms in precipitate have a relatively narrow size distribution. The average diameter of spherical ones is 30 ± 2 nm. The nanoprisms are 140 ± 25 nm in size along their longest edge and 8 ± 2 nm in thickness. As counted from TEM images, the number density of the collected nanoprisms is about 97%. Fig. 2c presents a typical HRTEM image of a prism with its basal plane lying on the carbon-coated copper grid. The well-resolved fringe pattern reveal the single crystalline of the gold product, which is also confirmed by the electron diffraction pattern (Fig. 2d) and the hexagonal symmetry of the diffracted spots suggests that the base faces of the gold nanoprism is bounded by {1 1 1} facets [19,20]. The fringes with a lattice spacing of 0.25 nm corresponds to the forbidden 1/3 {4 2 2} reflections [24], which has been ascribed to the existence of stacking fault parallel to the {1 1 1} facets [25]. At present time, we have not yet observed the stacking faults from these very thin prisms. However, in
Fig. 1. In situ UV–vis–NIR absorption spectra recording the growth process of gold nanoprisms after seed addition.
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the control experiment without adding I, quite a few of ellipsoids instead of prisms were produced because of the quick growth rate along the h1 1 1i direction of gold and we did discover the presence of stacking faults extending across the entire ellipsoid (Fig. 2e). Fig. 2f is the typical powder XRD pattern of the as-collected gold nanoprisms. The overwhelmingly intensive diffraction located at 2h = 38.12° corresponds to the {1 1 1} facets of face-centered cubic gold (JCPDS No. 4-0784), while peaks belonging to other facets are quite weak. These observations indicate their basal {1 1 1} facets typically lie flat on the supporting surface. It is well-known that colloidal gold usually exhibit an intense color because that the SPR frequency locates at the visible part of the spectrum. Fig. 3a and b show the photographs of the resulting spherical nanoparticle solution and the nanoprism solution, respectively. The supernatant consisting of spherical gold nanoparticles is commonly claret-red. Interestingly, the high-purity gold nanoprism solution exhibits green in color due to the different SPR frequency. It is also documented that the shape and the size of gold nanostructures play important roles in determining the number, position, and intensity of SPR modes [2,26]. As shown in Fig. 3c and d, the supernatant mainly containing spherical gold nanoparticles exhibits a single SPR band around 530 nm, while the gold nanoprism solution presents two SPR bands as a result of their high anisotropy in shape: one weak peak located at 760 nm and another strong, sharp peak centered at 1055 nm, which can be assigned to the in-plane quadrupole resonance and the in-plane dipole resonance, respectively [23]. The absence of any feature of this spectrum in the range of 500–600 nm is obvious, which implies that the amount of spherical particles is very minimal, providing another piece of evidence for the high-purity of these gold nanoprisms. Further investigation reveals that the significant aggregation of gold nanoprisms occurs only when the concentration of CTABr is greater than or equal to 0.05 M. The less CTABr is employed, the less precipitation of gold nanoprisms is observed. In a control experiment using 0.0125 M CTABr, the produced gold nanostructures were stable enough that no aggregation was observed. The corresponding UV–vis–NIR spectrum exhibits the weak quadrupole and strong dipole resonance bands, confirming the presence of plentiful gold nanoprisms (Fig. 4a). Subsequent TEM observation reveals that these gold nanostructures are mainly triangular nanoprisms accompanied with spherical ones (Fig. 5a). To achieve high-purity gold nanoprisms at a lower concentration of CTABr, we made the effort by adding NaCl instead of CTABr to the reaction mixture in order to increase the ionic concentration (detail described in Section 2.3). The experimental results clearly indicate that the aggregation and subsequent precipitation of the gold nanoprisms takes place after introducing NaCl (Figs. 4b and 5b) whereas the spherical ones are still kept in the solution (Fig. 4c). What is the reason for the significantly different stability between spherical nanoparticles and nanoprisms? Generally, the stability of gold nanostructures solution by the CTABr-assisted approach is dominated by two key factors: electrostatic repulsion between the CTA+ bilayer along the gold surface and the steric exclusion, both of which attribute to the cationic surfactant CTABr [21,27–29]. Meanwhile, the anions in the reaction solution, such as Br, would electrostatically bind to the cationic surface of the gold nanostructures and the overall positive charge of the gold nanostructures decreases. By partially shielding the positively charged gold surface with Br, the electrostatic repulsion among the gold nanostructures is decreased. Once the concentration of Br is high enough that the decrease of the positive charges approximates a critical point, the effect of electrostatic repulsion could be overcome, thus inducing the gold nanostructures to reach a close distance in which aggregation might occur. Similarly, post-adding anions (such as Cl) in the solution with a lower concentration of
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Fig. 2. TEM images of the gold nanostructures obtained in: (a) supernatant and (b) precipitate, respectively; initial [CTABr] is 0.05 M and aging time is 24 h. The inset in panel b shows a stack formed by gold nanoprisms standing perpendicularly to the TEM grid, which allows the precise measurement of their thickness. Scale bar = 20 nm. (c) HRTEM image and (d) SAED pattern of a triangular gold nanoprism. The spots marked using box, circle and triangle correspond to 1/3 {4 2 2}, {2 2 0} and {4 2 2} diffractions, respectively. (e) Gold nanoellipsoids obtained in the absence of I. The stacking faults in the ellipsoids are labeled by arrows. (f) XRD pattern taken from the as-collected gold nanoprisms.
Fig. 3. Digital photographs of: (a) spherical gold nanoparticles in supernatant and (b) as-collected gold nanoprisms in water, respectively. (c) and (d) show the corresponding UV–vis–NIR absorption spectra of (a) and (b), respectively.
Fig. 4. UV–vis–NIR absorption spectra of the reaction mixture before (a) and after (b) treatment by adding NaCl. (c) Spectrum of as-collected gold nanoprisms solution; initial [CTABr] is 0.0125 M.
CTABr is also able to trigger this electrostatic aggregation. On the other hand, the shape effect of the gold nanostructures is another important factor in effecting their aggregation potential. For spherical nanoparticles, their high-curvature geometry allows to have a
minimal contact with any shaped particles. However, for nanoprisms, their two-dimensional, nearly zero-curvature geometry leads to a maximal contact with each other. Thus, nanoprisms hold much higher aggregation potential than spherical ones when the
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Fig. 5. TEM images of: (a) reaction mixtures consist of spherical gold nanoparticles and gold triangular nanoprisms and (b) redispersed gold triangular nanoprisms. Initial [CTABr] is 0.0125 M.
positively charged gold surfaces are largely shielded by the concentrated anions. As a result, the aggregation and subsequent sedimentation of gold nanoprisms occurs directly while keeping most of the spherical ones in solution. Upon dispersing the precipitate of gold nanoprisms by water, the anions binding on the cationic gold surface would undergo a dissociation process to the solution until diffusive equilibrium, which recovers the function of CTA+ bilayer and allows these gold nanoprisms to form colloid again (Fig. 6). In addition, it has been noticed that when there is no extra CTABr in the solution, these gold nanoprisms undergo a gradual shape reconstruction process arising from the high-energy sharp corners of these thermodynamics-unfavored nanostructures [30]. This process results in blunted edges of gold nanoprisms and dramatic blue shift of their SPR band by several hundred nanometers (see Supporting information). Hence it is highly recommended that the as-collected gold nanoprisms should be stored in 5–10 mM CTABr solution for maintaining their shape as well as optical characters. The presented strategy is also flexible and effective for collecting much smaller gold nanoprisms in good-purity. In our further research, the growth of gold nanoprisms with the edge size of less than 100 nm has been developed by increasing the initial amount of seeds and I while keeping other synthetic parameters constant. As shown in Fig. 7a, the UV–vis–NIR spectrum of the resulting reaction mixture (curve1) reveals that the SPR band maximum of the
Fig. 7. (a) UV–vis–NIR absorption spectra of the reaction mixture before (curve 1) and after (curve 2) treatment using NaCl. Curve 3 shows the spectrum obtained from the aqueous suspension of as-collected gold nanoprisms. (b) TEM image of the as-collected gold nanoprisms. (c) Digital photograph of the aqueous suspension of gold nanoprisms. Initial [CTABr] is 0.05 M. The amount of iodide ion and seed is five times of that in typical samples, respectively. The reaction mixture was kept for 12 h, followed by adding NaCl until the sum of [Cl] and [Br] reached 1.0 M. Then the solution was aged for another 12 h before collecting the gold nanoprisms.
Fig. 6. A schematic illustration of achieving high-purity colloidal gold nanoprisms through a seed-mediated, iodide ion- and CTABr-assisted synthetic system.
nanoprisms dramatically blue shifted to 750 nm when the addition of seeds and I are five times of that in typical samples. Subsequent
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spectroscopy observation clearly reveals that the sedimentation of most of the gold nanoprisms after introducing sufficient NaCl (curves 2 and 3 in Fig. 7a). TEM observation displays these as-collected gold nanoprisms hold an average edge size of less than 50 nm (Fig. 7b) and their aqueous solution exhibits blue in color due to their ultra-small size (Fig. 7c). It has experimentally confirmed that in-plane plasmon oscillation of silver nanoprisms is extremely sensitive to environmental or aggregative changes [31–33]. Therefore, silver nanoprisms could be envisaged as plasmonic platforms for sensing applications. Based on the similarity of SPR modes in shape and intensities between gold and silver nanoprisms by theoretical studies [23], it is reasonably believed that gold nanoprisms also hold the capability for plasmonic sensing platforms. In this research we chose human cardiac troponin I (cTnI), a highly specific biomarker in the clinic diagnosis of myocardial infarction [34], as a protein sensing model based on the SPR changes of gold nanoprisms in solution. To the best of our knowledge, there have no previous reports on using noble metal nanoprisms as plasmonic biosensors. Although there are several publications have demonstrated that the washed CTABr-capped gold nanostructrues are nontoxic and able to conjugate antibody protein directly [35,36], our experiments reveal that the CTABr on the gold nanoprisms indeed cause the denaturation and sedimentation of anti-cTnI antibodies. Therefore, anionic polyelectrolyte PSS has been firstly coated on the positively charged surface of gold nanoprisms by electrostatic interaction for passivating the cytotoxicity of CTABr and keeping optical stability. The subsequent conjugation of the antibody on the PSS-coated gold nanoprisms takes advantage of two interactions: (1) electrostatic interaction between the negative charged PSS and the positively charged segment of the antibody; (2) hydrophobic interaction between the aryl portion of PSS and the hydrophobic segment of the antibody [37,38]. As shown in Fig. 8, the in-plane dipole resonance absorption maximum has a 5 nm red-shift after PSS coating. After the anti-cTnI antibody conjugation, there is a 10 nm red-shift of the in-plane dipole resonance absorption maximum. No shift in the absorption maximum of the in-plane quadrupole resonance is also observed during the whole process, indicating that the in-plane dipole resonance of gold nanoprisms is quite sensitive to the changes of the local dielectric surrounding. Further evidence for CTABr-capped gold nanoprisms coated with PSS and subsequently anti-cTnI antibody can be seen from the gradual changes of corresponding FTIR spectra (see Supporting information). Fig. 9 shows the changes in the SPR band of the anti-cTnI antibody conjugated gold nanoprism
Fig. 9. UV–vis–NIR absorption spectra for anti-cTnI antibody conjugated gold nanoprims before (a) and after (b)–(h) treatment with cTnI (10, 50, 100, 300, 500, 700, 1000 ng/mL in TBS buffer), respectively. Inset is the enlarged section of the spectra shown in the dash box.
solution after adding cTnI with increasing concentrations (10– 1000 ng/ml). It can be seen the in-plane dipole resonance band continuously decreases in intensity with a small red-shift, which can be assigned to the aggregation of gold nanoprisms driven by the specific binding events between anti-cTnI antibody and cTnI. The above experimental result is somewhat different from the sensing based on analyte-mediated spherical gold nanoparticles aggregation, in which the SPR peak not only gradually diminish but also undergo an apparent red-shift process due to plasmon coupling of particle–particle aggregates [39]. However, it is worth mentioning that Jain et al. previously reported that the shape anisotropy of gold nanorods can leave them to be aggregated into two distinct orientational modes, namely end-to-end and side-byside, and optical experiments revealed that side-by-side linkage of nanorods in solution exhibits a blue-shift of the longitudinal plasmon band while end-by-end linkage gives a red-shift [40]. Recently, Sendroiu et al. also observed that the formation of random- aggregates of gold nanorods upon DNA hybridization mainly induce the decrease of the SPR band intensity [41]. We thus presume that adding CTnI to the anti-cTnI antibody conjugated gold nanoprism solution could induce the nanoprisms to aggregate in a random-orientation mode, resulting in an obvious decreasing intensity of in-plane dipole resonance band accompanied with a slight red-shift. Our experimental results indicate that the sensing of cTnI is achieved with a sensitivity of ca. 50 ng/mL according to the appearance of an obvious decrease in intensity of in-plane dipole resonance band, which is comparable to that of gold nanorods used as plasmonic platform in solution [42,43]. Moreover, control experiments indicate that no SPR changes have been observed when the anti-cTnI antibody conjugated gold nanoprism solution was used to detect other biomarkers instead of cTnI.
4. Summary
Fig. 8. UV–vis–NIR absorption spectra of gold nanoprisms at different stages of surface functionalization: (a) as-collected CTABr-capped gold nanoprisms; (b) PSScoated gold nanoprisms and (c) anti-cTnI antibody conjugated gold nanoprisms.
In conclusion, we have demonstrated that colloidal gold triangular nanoprisms with high-purity (97%) can be achieved conveniently by taking advantage of the electrostatic aggregation and shape effects through a seed-mediated, iodide ion- and CTABrassisted synthetic system. The separation process between the produced gold nanoprisms and spherical gold nanoparticles can be easily triggered by adjusting the anion concentration in the reaction solution, without the need of centrifugation or filtration. Further study confirms that this strategy is effective while flexible
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enough that ultra-small gold nanoprisms can also been achieved in good-purity. These as-collected gold nanoprism solutions are shown to possess a remarkably enhanced performance in surface plasmon resonance and NIR absorbing compared with the original mixtures. Furthermore, investigation on their potential as biosensing platforms based on the changes of the SPR band induced by the specific antibody–antigen recognition events clearly indicates that gold nanoprisms will be a promising candidate for solutionbased plasmonic sensing. In addition, these colloidal gold nanoprisms, with extremely flat surface and strong NIR absorption, could also find practical applications for drug delivery and hyperthermia treatment of tumors in vivo.
Acknowledgments Financial assistance from the High-technology Platform of Jiangsu Province for Molecular Diagnosis and Biological Therapy of Critical Illness (XK200705), the National Natural Science Foundation of China (Nos. 30870679 and 30970787) and China Postdoctoral Science Foundation (20090451236) is gratefully acknowledged. Thanks also to Dr. M. Wang and Dr. L. Huang for help with preparation of the manuscript.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2010.04.013. Additional TEM images, FTIR and UV–vis–NIR absorption spectra.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Achieving high-purity colloidal gold nanoprisms and their application as biosensing platforms Zhirui Guo a, Xu Fan b, Lianke Liu a, Zhiping Bian a, Chunrong Gu a, Yu Zhang b, Ning Gu b, Di Yang a,*, Jinan Zhang a,** a b
Institute of Cardiovascular Disease, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China State Key Laboratory of Bioelectronics and Jiangsu Laboratory for Biomaterials and Devices, Southeast University, Nanjing 210096, China
a r t i c l e
i n f o
Article history: Received 9 December 2009 Accepted 9 April 2010 Available online 18 May 2010 Keywords: Gold Nanostructure Electrostatic interaction Anisotropic shape Biosensor
a b s t r a c t Gold nanoprisms with average edge size of 140 nm and thickness of 8 nm were achieved in highpurity (97%) by exploiting the electrostatic aggregation and shape effects through a modified seedmediated approach. The proposed strategy lies in the dramatically different stability and aggregation potential between the produced gold nanoprisms and spherical gold nanoparticles, which can be modulated by varying the anion concentration in the reaction solution. Hence, the gold nanoprisms spontaneously aggregated into precipitate whereas most of the spherical ones were still kept in the solution. Moreover, this strategy is also flexible enough that ultra-small gold nanoprisms with average width less than 50 nm can be collected in good-purity. The structure and optical properties of these nanoprisms have been studied by TEM, SAED, XRD and UV–vis–NIR spectroscopy, respectively. These high-purity colloidal gold nanoprisms exhibit remarkably enhanced surface plasmon resonance (SPR) as well as strong near-infrared absorption. Furthermore, we have also investigated their potential for biosensing based on the sensitive changes of SPR band induced by the antibody–antigen recognition events. The experimental results clearly suggest that gold nanoprisms can be a promising nanostructured system for plasmonic sensor applications. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Finely controlling the shape and size of metal nanostructures has become an active area of research because the optical, electrical, magnetic and catalytic properties of metal nanostructures are strongly dependent on their shape and size [1,2]. Thanks to the efforts of the worldwide research groups, multiplex shaped metal nanostructures, including icosahedras [3], decahedrons [4], octahedrons [5], bipyramids [6], cubes [7], rods [8], wires [9], belts (ribbons) [10], and other complex shapes such as multipods [11] or stars [12], have been synthesized during the last decade. In recent few years, gold and silver nanoprisms (nanoplates) have attracted great interest because of their unique optical properties known as localized surface plasmon resonance (SPR) and promising applications in photoluminescence [13], surface-enhanced Raman scattering [14], optical coating [15] and near-infrared (NIR) light absorbing [16]. In general, noble metal nanoprisms are a class of two-dimensional structures with parallel top and bottom and a nanoscale thickness. Although micrometer-sized gold or silver
* Corresponding author. Fax: +86 25 83738572. ** Corresponding author. Fax: +86 25 83738572. E-mail addresses: [email protected] (D. Yang), (J. Zhang).
[email protected]
0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.04.013
prisms with thickness below 100 nm are commonly categorized as nanoprisms, they have not presented the unique optical properties associated with their colloidal analogs [17]. Furthermore, the solution of micrometer-sized prisms is prone to precipitate due to their large size, limiting the applications in solution phase. Up to date, a variety of synthetic strategies, such as photochemical, thermal and biological routes, have been explored to prepare bulk quantities of colloidal silver nanoprisms [18]. However, most of the synthesized gold nanoprisms have edge lengths ranging from submicrometer to tens of micrometer [19]. A few reports concerned with small gold nanoprisms often generate large amounts of other shaped structures difficult to high-efficiently separate [20]. Because gold is much more air-stable and biocompatible than silver, the achievement of colloidal gold nanoprisms with high-purity are quite desirable for expanding the application field which is presently unattainable by silver nanoprisms. Recently, Ha et al. reported the synthesis of small gold nanoprisms by a seed-mediated, cetyltrimethylammonium bromide (CTABr)-assisted approach [21]. In their work, a proper amount of iodide ion (I) was introduced into the reaction solution to repress the crystal growth along the h1 1 1i direction of gold, resulting in basal {1 1 1} facets bounded triangular gold nanoprisms. However, the final reaction mixture is still accompanied with a large fraction (55%) of spherical nanoparticles. In this paper we
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describe our recent research on the stability and aggregation potential between the gold nanoprisms and the spherical gold nanoparticles produced by the above-mentioned seeding synthetic system. It is found that colloidal gold nanoprisms with an excellent purity (97% in number density) can be achieved from the in situ reaction mixtures by exploiting the electrostatic aggregation effect, without conventional centrifugation or filtration [21,22]. The presented strategy is effective while flexible enough that ultra-small gold nanoprisms with average edge length less than 50 nm can be obtained in good-purity. Moreover, their SPR properties is studied and the potential as biosensing platforms using biomarker human cardiac troponin I as a protein sensing model is investigated. The experimental facts clearly demonstrate that gold nanoprisms can be a promising candidate for plasmonic sensor applications. 2. Materials and methods 2.1. Materials Cetyltrimethylammonium bromide (CTABr)1 and Poly(styrenesulphonate) (PSS, Mw 70,000) were purchased from Sigma. Tris(hydroxymethylamino)methane was received from Amresco. HAuCl4, sodium citrate, NaBH4, L-ascorbic acid (AA), NaOH, KI, K2CO3 and NaCl were all purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd. (China). All reagents were used as received without further purification. Millipore-quality water (18.2 MX cm) was used throughout the experiments. Tris Buffered Saline (TBS buffer) was prepared by dissolving 2.42 g tris(hydroxymethylamino)methane and 8.8 g NaCl in 900 mL of water. The solution was titrated with 1.00 M HCl until a constant pH value of 8.2 was obtained, and then water was added to receive a 1000 mL stock solution. Human cardiac troponin I (cTnI) from human myocardium muscle and the anti-human cTnI antibodies used in the experiment were obtained from Research Institute of Cardiovascular Disease of First Affiliated Hospital of Nanjing Medical University. All the glassware was cleaned by aqua regia (HCl:HNO3 in a 3:1 ratio by volume) and rinsed with water prior to the experiments. 2.2. Synthesis of gold seeds Gold seeds were synthesized by reducing 0.4 mL of 25 mM HAuCl4 with 1 mL of ice-cold 0.1 M NaBH4 with vigorously stirring. The reduction was done in the presence of 1 mL of 10 mM sodium citrate and 37.6 mL of water. Upon addition of the NaBH4, the solution turned a reddish orange color, indicating the generation of gold nanoparticles. The resulting solution was continually stirred for 2 min. The seed solution was allowed to stand for at least 2 h to ensure the complete hydrolysis of unreacted NaBH4. The gold seeds exhibited a SPR peak at 507 nm, and had an average diameter of less than 5.0 nm (see Supporting information). 2.3. Growth and achievement of gold nanoprisms with high-purity An initial reaction mixture of gold nanoprisms was prepared by a one-step seed-mediated, iodide ion- and CTABr-assisted approach with new modifications [21,22]. Typically, a 100 mL of growth solution containing 0.25 mM HAuCl4 and 0.05 M CTABr was prepared in a 150 mL beaker. To this solution was added 55 ll of 0.1 M KI, 1 Researchers have recently reported that the iodide ion impurity concentrations in CTABr vary from different manufacturers and the range is from less than 2.75 ppm to 840 ppm [44]. In current work, the needing [I] is greater than or equal to 50 lM. Thus to eliminate the side effects by the iodide ion impurity in CTABr, we have chosen Cat No: H6269 from Sigma, which contains the least iodide concentration (0.37 lM in our reaction solution) among the CTABrs.
0.55 mL of 0.1 M AA and 0.55 mL of 0.1 M NaOH (NaOH is used to deprotonate ascorbic acid for more effective reducibility) in turn and the resulting solution as growth solution was stirred gently. The orange color of the gold salt in the CTABr solution disappeared when AA was added, due to the reduction of Au3+ to Au+. The growth of gold nanoprisms was initiated by adding 0.1 mL of the seed solution to the growth solution. After the addition, the color of the growth solution changed from clear to light red, and then turned to deep claret-red over a period of 30 min. The reaction solution was kept at 30 °C and left undisturbed during the whole aging period. There were two ways for achieving high-purity colloidal gold nanoprisms: when the concentration of CTABr was greater than or equal to 0.05 M, the reaction mixture was aged without disturbance for 24 h; when the concentration of CTABr was lesser than 0.05 M, the reaction mixture was aged for 12 h, followed by adding NaCl until the sum of [Cl] and [Br] reached 0.2 M. After this, the reaction mixture was aged for another 12 h. Most of gold nanoprisms deposited on the bottom of the beaker during the aging period, and were easily collected by pouring out the supernatant. These gold products were redispersed to the colloidal solution for further characterization. 2.4. Conjugation of gold nanoprisms to anti-cTnI antibodies The high-purity nanoprisms in the CTABr solution was centrifuged to get rid of the extra free CTABr molecules. The nanoprism surface was coated with PSS polyelectrolyte before antibody conjugation. This was done by adding 2.4 ml of 2.5 mg/mL PSS to 30 mL nanoprism solution with an optical density of 1.0 at the long wavelength absorption maximum. The reaction was continued for about 15 min to complete the coating process. The extra PSS in solution was separated by centrifuging the prism solution at 3000 rpm. The pellet was redispersed in water and titrated with 0.2 M K2CO3 to reach a pH value of 8.2. Then the PSS capped nanoprism solution was mixed with an excess amount of anti-cTnI antibody (50 lg/mL) to react for 30 min. The nanoprisms conjugated with the anti-cTnI antibodies were centrifuged twice and redispersed into TBS buffer to form a stock solution with an optical density of around 1.0 at the long wavelength absorption maximum (washing buffer: 1% bovine serum albumin in TBS buffer to preclude nonspecific binding). 2.5. Binding of cTnI to anti-cTnI antibody conjugated gold nanoprisms In each experiment, 1.0 mL of anti-cTnI antibody conjugated gold nanoprisms was added to a fixed amount (10, 50, 100, 300, 500, 700 and 1000 ng) of cTnI at room temperature under vortex mixing. The resultant mixture was incubated for 15 min before recording with 2802S (UNICO) spectrophotometer. 2.6. Characterization The morphologies of the gold products were characterized by TEM (JEM-2000EX, JEOL) operating at 120 kV. The high-resolution TEM (HRTEM) image and the selected area electron diffraction (SAED) pattern were recorded on a JEOL JEM-2100 transmission electron microscope operating at 200 kV. The XRD analysis of the gold samples was carried out by using an X-ray diffractometer (D/Max-RA, Rigaku) with Cu Ka radiation at room temperature. The sample was deposited onto a silicon wafer and scanned in the 2h range of 30–90°. The Fourier transform infrared (FTIR) spectra were measured on a Magna FTIR-750 (Nicolet) spectrometer and the vacuum-dried samples were made in the form of KBr pellet. The UV–vis–NIR absorption spectra of the gold samples were collected by a UNICO 2802S spectrophotometer in a wavelength range from 300 to 1100 nm.
Z. Guo et al. / Journal of Colloid and Interface Science 348 (2010) 29–36
3. Results and discussion The key point of our strategy lies in the different stability and aggregation potential between spheres and prisms. In a typical experiment conducted in the presence of 0.05 M CTABr, the growth of gold nanostructures has been monitored by UV–vis–NIR spectroscopy in real time. As shown in Fig. 1, two distinct SPR bands emerge within 2 min after seed addition. The first one appears at 532 nm and its intensity increases with time, which indicates that a large number of spherical gold nanoparticles are produced accompanying the growth of nanoprisms. The second band initially appears at 930 nm and this band red-shifts with the increase of the intensity as the growth continues, which indicates the presence of gold nanoprisms and the band shifts to NIR region could mirror an increase in edge length [23]. After 10 min, the intensity of the SPR band assigned to nanoprisms reaches its maximum and, unexpectly, begins to decrease gradually. At the meantime, the SPR band of spherical ones still keeps increasing in intensity. At the end of the aging period, the SPR band of gold nanoprisms disappears completely and only the SPR band of spherical ones is left, which reveals that the generated gold nanoprisms are much less stable than spherical ones and most of them aggregate spontaneously into precipitation. This precipitation can be collected easily by pouring out the supernatant. In particular, these as-obtained gold nanoprisms are prone to form colloidal solution by redispersion in water. As shown in Fig. 2a and b, both the spherical nanoparticles in supernatant and the triangular nanoprisms in precipitate have a relatively narrow size distribution. The average diameter of spherical ones is 30 ± 2 nm. The nanoprisms are 140 ± 25 nm in size along their longest edge and 8 ± 2 nm in thickness. As counted from TEM images, the number density of the collected nanoprisms is about 97%. Fig. 2c presents a typical HRTEM image of a prism with its basal plane lying on the carbon-coated copper grid. The well-resolved fringe pattern reveal the single crystalline of the gold product, which is also confirmed by the electron diffraction pattern (Fig. 2d) and the hexagonal symmetry of the diffracted spots suggests that the base faces of the gold nanoprism is bounded by {1 1 1} facets [19,20]. The fringes with a lattice spacing of 0.25 nm corresponds to the forbidden 1/3 {4 2 2} reflections [24], which has been ascribed to the existence of stacking fault parallel to the {1 1 1} facets [25]. At present time, we have not yet observed the stacking faults from these very thin prisms. However, in
Fig. 1. In situ UV–vis–NIR absorption spectra recording the growth process of gold nanoprisms after seed addition.
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the control experiment without adding I, quite a few of ellipsoids instead of prisms were produced because of the quick growth rate along the h1 1 1i direction of gold and we did discover the presence of stacking faults extending across the entire ellipsoid (Fig. 2e). Fig. 2f is the typical powder XRD pattern of the as-collected gold nanoprisms. The overwhelmingly intensive diffraction located at 2h = 38.12° corresponds to the {1 1 1} facets of face-centered cubic gold (JCPDS No. 4-0784), while peaks belonging to other facets are quite weak. These observations indicate their basal {1 1 1} facets typically lie flat on the supporting surface. It is well-known that colloidal gold usually exhibit an intense color because that the SPR frequency locates at the visible part of the spectrum. Fig. 3a and b show the photographs of the resulting spherical nanoparticle solution and the nanoprism solution, respectively. The supernatant consisting of spherical gold nanoparticles is commonly claret-red. Interestingly, the high-purity gold nanoprism solution exhibits green in color due to the different SPR frequency. It is also documented that the shape and the size of gold nanostructures play important roles in determining the number, position, and intensity of SPR modes [2,26]. As shown in Fig. 3c and d, the supernatant mainly containing spherical gold nanoparticles exhibits a single SPR band around 530 nm, while the gold nanoprism solution presents two SPR bands as a result of their high anisotropy in shape: one weak peak located at 760 nm and another strong, sharp peak centered at 1055 nm, which can be assigned to the in-plane quadrupole resonance and the in-plane dipole resonance, respectively [23]. The absence of any feature of this spectrum in the range of 500–600 nm is obvious, which implies that the amount of spherical particles is very minimal, providing another piece of evidence for the high-purity of these gold nanoprisms. Further investigation reveals that the significant aggregation of gold nanoprisms occurs only when the concentration of CTABr is greater than or equal to 0.05 M. The less CTABr is employed, the less precipitation of gold nanoprisms is observed. In a control experiment using 0.0125 M CTABr, the produced gold nanostructures were stable enough that no aggregation was observed. The corresponding UV–vis–NIR spectrum exhibits the weak quadrupole and strong dipole resonance bands, confirming the presence of plentiful gold nanoprisms (Fig. 4a). Subsequent TEM observation reveals that these gold nanostructures are mainly triangular nanoprisms accompanied with spherical ones (Fig. 5a). To achieve high-purity gold nanoprisms at a lower concentration of CTABr, we made the effort by adding NaCl instead of CTABr to the reaction mixture in order to increase the ionic concentration (detail described in Section 2.3). The experimental results clearly indicate that the aggregation and subsequent precipitation of the gold nanoprisms takes place after introducing NaCl (Figs. 4b and 5b) whereas the spherical ones are still kept in the solution (Fig. 4c). What is the reason for the significantly different stability between spherical nanoparticles and nanoprisms? Generally, the stability of gold nanostructures solution by the CTABr-assisted approach is dominated by two key factors: electrostatic repulsion between the CTA+ bilayer along the gold surface and the steric exclusion, both of which attribute to the cationic surfactant CTABr [21,27–29]. Meanwhile, the anions in the reaction solution, such as Br, would electrostatically bind to the cationic surface of the gold nanostructures and the overall positive charge of the gold nanostructures decreases. By partially shielding the positively charged gold surface with Br, the electrostatic repulsion among the gold nanostructures is decreased. Once the concentration of Br is high enough that the decrease of the positive charges approximates a critical point, the effect of electrostatic repulsion could be overcome, thus inducing the gold nanostructures to reach a close distance in which aggregation might occur. Similarly, post-adding anions (such as Cl) in the solution with a lower concentration of
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Fig. 2. TEM images of the gold nanostructures obtained in: (a) supernatant and (b) precipitate, respectively; initial [CTABr] is 0.05 M and aging time is 24 h. The inset in panel b shows a stack formed by gold nanoprisms standing perpendicularly to the TEM grid, which allows the precise measurement of their thickness. Scale bar = 20 nm. (c) HRTEM image and (d) SAED pattern of a triangular gold nanoprism. The spots marked using box, circle and triangle correspond to 1/3 {4 2 2}, {2 2 0} and {4 2 2} diffractions, respectively. (e) Gold nanoellipsoids obtained in the absence of I. The stacking faults in the ellipsoids are labeled by arrows. (f) XRD pattern taken from the as-collected gold nanoprisms.
Fig. 3. Digital photographs of: (a) spherical gold nanoparticles in supernatant and (b) as-collected gold nanoprisms in water, respectively. (c) and (d) show the corresponding UV–vis–NIR absorption spectra of (a) and (b), respectively.
Fig. 4. UV–vis–NIR absorption spectra of the reaction mixture before (a) and after (b) treatment by adding NaCl. (c) Spectrum of as-collected gold nanoprisms solution; initial [CTABr] is 0.0125 M.
CTABr is also able to trigger this electrostatic aggregation. On the other hand, the shape effect of the gold nanostructures is another important factor in effecting their aggregation potential. For spherical nanoparticles, their high-curvature geometry allows to have a
minimal contact with any shaped particles. However, for nanoprisms, their two-dimensional, nearly zero-curvature geometry leads to a maximal contact with each other. Thus, nanoprisms hold much higher aggregation potential than spherical ones when the
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33
Fig. 5. TEM images of: (a) reaction mixtures consist of spherical gold nanoparticles and gold triangular nanoprisms and (b) redispersed gold triangular nanoprisms. Initial [CTABr] is 0.0125 M.
positively charged gold surfaces are largely shielded by the concentrated anions. As a result, the aggregation and subsequent sedimentation of gold nanoprisms occurs directly while keeping most of the spherical ones in solution. Upon dispersing the precipitate of gold nanoprisms by water, the anions binding on the cationic gold surface would undergo a dissociation process to the solution until diffusive equilibrium, which recovers the function of CTA+ bilayer and allows these gold nanoprisms to form colloid again (Fig. 6). In addition, it has been noticed that when there is no extra CTABr in the solution, these gold nanoprisms undergo a gradual shape reconstruction process arising from the high-energy sharp corners of these thermodynamics-unfavored nanostructures [30]. This process results in blunted edges of gold nanoprisms and dramatic blue shift of their SPR band by several hundred nanometers (see Supporting information). Hence it is highly recommended that the as-collected gold nanoprisms should be stored in 5–10 mM CTABr solution for maintaining their shape as well as optical characters. The presented strategy is also flexible and effective for collecting much smaller gold nanoprisms in good-purity. In our further research, the growth of gold nanoprisms with the edge size of less than 100 nm has been developed by increasing the initial amount of seeds and I while keeping other synthetic parameters constant. As shown in Fig. 7a, the UV–vis–NIR spectrum of the resulting reaction mixture (curve1) reveals that the SPR band maximum of the
Fig. 7. (a) UV–vis–NIR absorption spectra of the reaction mixture before (curve 1) and after (curve 2) treatment using NaCl. Curve 3 shows the spectrum obtained from the aqueous suspension of as-collected gold nanoprisms. (b) TEM image of the as-collected gold nanoprisms. (c) Digital photograph of the aqueous suspension of gold nanoprisms. Initial [CTABr] is 0.05 M. The amount of iodide ion and seed is five times of that in typical samples, respectively. The reaction mixture was kept for 12 h, followed by adding NaCl until the sum of [Cl] and [Br] reached 1.0 M. Then the solution was aged for another 12 h before collecting the gold nanoprisms.
Fig. 6. A schematic illustration of achieving high-purity colloidal gold nanoprisms through a seed-mediated, iodide ion- and CTABr-assisted synthetic system.
nanoprisms dramatically blue shifted to 750 nm when the addition of seeds and I are five times of that in typical samples. Subsequent
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spectroscopy observation clearly reveals that the sedimentation of most of the gold nanoprisms after introducing sufficient NaCl (curves 2 and 3 in Fig. 7a). TEM observation displays these as-collected gold nanoprisms hold an average edge size of less than 50 nm (Fig. 7b) and their aqueous solution exhibits blue in color due to their ultra-small size (Fig. 7c). It has experimentally confirmed that in-plane plasmon oscillation of silver nanoprisms is extremely sensitive to environmental or aggregative changes [31–33]. Therefore, silver nanoprisms could be envisaged as plasmonic platforms for sensing applications. Based on the similarity of SPR modes in shape and intensities between gold and silver nanoprisms by theoretical studies [23], it is reasonably believed that gold nanoprisms also hold the capability for plasmonic sensing platforms. In this research we chose human cardiac troponin I (cTnI), a highly specific biomarker in the clinic diagnosis of myocardial infarction [34], as a protein sensing model based on the SPR changes of gold nanoprisms in solution. To the best of our knowledge, there have no previous reports on using noble metal nanoprisms as plasmonic biosensors. Although there are several publications have demonstrated that the washed CTABr-capped gold nanostructrues are nontoxic and able to conjugate antibody protein directly [35,36], our experiments reveal that the CTABr on the gold nanoprisms indeed cause the denaturation and sedimentation of anti-cTnI antibodies. Therefore, anionic polyelectrolyte PSS has been firstly coated on the positively charged surface of gold nanoprisms by electrostatic interaction for passivating the cytotoxicity of CTABr and keeping optical stability. The subsequent conjugation of the antibody on the PSS-coated gold nanoprisms takes advantage of two interactions: (1) electrostatic interaction between the negative charged PSS and the positively charged segment of the antibody; (2) hydrophobic interaction between the aryl portion of PSS and the hydrophobic segment of the antibody [37,38]. As shown in Fig. 8, the in-plane dipole resonance absorption maximum has a 5 nm red-shift after PSS coating. After the anti-cTnI antibody conjugation, there is a 10 nm red-shift of the in-plane dipole resonance absorption maximum. No shift in the absorption maximum of the in-plane quadrupole resonance is also observed during the whole process, indicating that the in-plane dipole resonance of gold nanoprisms is quite sensitive to the changes of the local dielectric surrounding. Further evidence for CTABr-capped gold nanoprisms coated with PSS and subsequently anti-cTnI antibody can be seen from the gradual changes of corresponding FTIR spectra (see Supporting information). Fig. 9 shows the changes in the SPR band of the anti-cTnI antibody conjugated gold nanoprism
Fig. 9. UV–vis–NIR absorption spectra for anti-cTnI antibody conjugated gold nanoprims before (a) and after (b)–(h) treatment with cTnI (10, 50, 100, 300, 500, 700, 1000 ng/mL in TBS buffer), respectively. Inset is the enlarged section of the spectra shown in the dash box.
solution after adding cTnI with increasing concentrations (10– 1000 ng/ml). It can be seen the in-plane dipole resonance band continuously decreases in intensity with a small red-shift, which can be assigned to the aggregation of gold nanoprisms driven by the specific binding events between anti-cTnI antibody and cTnI. The above experimental result is somewhat different from the sensing based on analyte-mediated spherical gold nanoparticles aggregation, in which the SPR peak not only gradually diminish but also undergo an apparent red-shift process due to plasmon coupling of particle–particle aggregates [39]. However, it is worth mentioning that Jain et al. previously reported that the shape anisotropy of gold nanorods can leave them to be aggregated into two distinct orientational modes, namely end-to-end and side-byside, and optical experiments revealed that side-by-side linkage of nanorods in solution exhibits a blue-shift of the longitudinal plasmon band while end-by-end linkage gives a red-shift [40]. Recently, Sendroiu et al. also observed that the formation of random- aggregates of gold nanorods upon DNA hybridization mainly induce the decrease of the SPR band intensity [41]. We thus presume that adding CTnI to the anti-cTnI antibody conjugated gold nanoprism solution could induce the nanoprisms to aggregate in a random-orientation mode, resulting in an obvious decreasing intensity of in-plane dipole resonance band accompanied with a slight red-shift. Our experimental results indicate that the sensing of cTnI is achieved with a sensitivity of ca. 50 ng/mL according to the appearance of an obvious decrease in intensity of in-plane dipole resonance band, which is comparable to that of gold nanorods used as plasmonic platform in solution [42,43]. Moreover, control experiments indicate that no SPR changes have been observed when the anti-cTnI antibody conjugated gold nanoprism solution was used to detect other biomarkers instead of cTnI.
4. Summary
Fig. 8. UV–vis–NIR absorption spectra of gold nanoprisms at different stages of surface functionalization: (a) as-collected CTABr-capped gold nanoprisms; (b) PSScoated gold nanoprisms and (c) anti-cTnI antibody conjugated gold nanoprisms.
In conclusion, we have demonstrated that colloidal gold triangular nanoprisms with high-purity (97%) can be achieved conveniently by taking advantage of the electrostatic aggregation and shape effects through a seed-mediated, iodide ion- and CTABrassisted synthetic system. The separation process between the produced gold nanoprisms and spherical gold nanoparticles can be easily triggered by adjusting the anion concentration in the reaction solution, without the need of centrifugation or filtration. Further study confirms that this strategy is effective while flexible
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enough that ultra-small gold nanoprisms can also been achieved in good-purity. These as-collected gold nanoprism solutions are shown to possess a remarkably enhanced performance in surface plasmon resonance and NIR absorbing compared with the original mixtures. Furthermore, investigation on their potential as biosensing platforms based on the changes of the SPR band induced by the specific antibody–antigen recognition events clearly indicates that gold nanoprisms will be a promising candidate for solutionbased plasmonic sensing. In addition, these colloidal gold nanoprisms, with extremely flat surface and strong NIR absorption, could also find practical applications for drug delivery and hyperthermia treatment of tumors in vivo.
Acknowledgments Financial assistance from the High-technology Platform of Jiangsu Province for Molecular Diagnosis and Biological Therapy of Critical Illness (XK200705), the National Natural Science Foundation of China (Nos. 30870679 and 30970787) and China Postdoctoral Science Foundation (20090451236) is gratefully acknowledged. Thanks also to Dr. M. Wang and Dr. L. Huang for help with preparation of the manuscript.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2010.04.013. Additional TEM images, FTIR and UV–vis–NIR absorption spectra.
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