Department of Chemical & Bio Engineering, Kyungwon University, Gyeonggi-do 461-701, Korea Received August 1, 2006; Accepted October 2, 2006
Abstract: Gold nanoshells are new constructional units consisting of dielectric core encapsulated by gold nano- layer that exhibits a plasmon-derived optical resonance typically shifted to near-infrared (NIR) wavelengths (800∼1200 nm). The fabrication of gold nanoshell was optimized based on the relative contributions of the aging of gold colloids, concentration of reducible gold salts, and dosages of formaldehyde reducing agents.
One-week-aged gold colloids produced a relatively monodisperse deposition of gold seeds onto silica nano- particles, whereas one-day-aged gold colloids produced a heterogeneous deposition of gold seeds on the silica nanoparticle surfaces. At the optimal dosages of reducing agents, the thickness of the gold layer increased with the increment of the concentrations of the reducible gold salts, consequently leading to the smoother morphol- ogies and stronger plasmon resonances of the gold nanoshells. The excessive addition of gold salts and/or re- ducing agents, however, resulted in a significant reduction of the plasmon resonance because of the increased aggregation between the gold nanoparticles.
Keywords: gold nanoshell, aging, near-infrared (NIR), plasmon resonance
Introduction
1)
Core-shell nanocomposites represent new types of con- structional units that are currently the subject of active re- search areae [1]. Surface plasmon resonance (SPR) is the term given to the collective oscillations of free electrons at the metallic interface surface. These oscillations can give rise to the intense colors of nanoparticle solutions because of their strong plasmon-derived absorption bands. A gold nanoshell is a layered nanoparticle consisting of a dielec- tric core surrounded by a gold metallic shell; these par- ticles exhibit a plasmon-derived optical resonance typi- cally shifted to much longer wavelength than the corre- sponding plasmon resonance of gold nanoparticles [2].
As shown in Figure 1, the fabrication of gold nanoshells consists of the following consecutive steps: i) synthesis of gold colloids and APTMS-functionalized silica nano- particles, ii) attachment of gold colloids onto the function- alized silica surface, and iii) gold shell growth over the silica cores. The silica surface must be modified with pos-
†To whom all correspondence should be addressed.
(e-mail: [email protected]. kr)
itively charged amino groups to avoid electrostatic re- pulsions between gold and silica nanoparticles having the same surface charges. After the gold colloids are attached onto the aminated silica surface, the gold-deposited silica (Au-APTMS/SiO2) is further reduced by reaction with gold salts in the presence of reducing agents. Finally, the continuous gold layer is formed over the silica cores via ucleate-induced cluster coalescence [2,3].
Gold nanoshells with strong near-infrared (NIR) absorp- tion can be applied to hyperthermia cancer therapy and photo-thermal-induced drug delivery because plasmon resonance at NIR wavelength exhibits optimal tissue transmissivity. Human breast carcinoma cells incubated with gold nanoshells have undergone photo-thermally in- duced morbidity on exposure to NIR light [3]. Yiwen Wang’s group explored the feasibility of using metal nanoshells as intravascular optical contrasts in a rat brain.
In contrast to NIR absorbing dyes, the optical properties of nanoshells, which are dependent upon a rigid metallic structure, are not susceptible to the photo-bleaching that is an inevitable problem when using organic dyes [4].
The practical applications of gold nanoshells, however, are limited by the penetration depth of the NIR laser (800
Figure 1. Schematic representation of gold layer formation over silica cores.
∼1200 nm) that is used to activate plasmon-derived local heating on the gold surface. In a recent study, Halas found that the temperature rise induced by the embedded gold nanoshells in deep tissues was relatively lower than those in shallow tissues [5]. The intensity and position of the NIR-absorption spectra determines the effectiveness of gold nanoshells in biomedical applications.
We have systematically investigated the relative con- tributions of various experimental parameters (such as ag- ing of gold colloids and the amounts of gold reducible salts and formaldehyde reducing agents) to the surface morphology and plasmon resonance of gold nanoshells during the nucleate-mediated fabrication process. The structural morphology and optical properties of gold nano- shells were characterized using TEM, SEM, and UV-Vis spectroscopy.
Experimental
Materials
All of the following reagents were purchased from Aldrich and used as received: 3-aminopropyl trimethox- ysilane (APTMS, 97 %), tetraethyl orthosilicate (TEOS, 99.999 %), tetrakis (hydroxymethyl) phosphonium chlor- ide (THPC, 80 % solution in water), ammonium hydrox- ide (NH4OH, 28 % NH3 in water), formaldehyde (HCHO,
Figure 2. Size variation of silica nanoparticles with respect to the ammonium ion dosages (28 wt%) at room temperature and 48
oC. The standard deviation of silica size is denoted by the error bars.
37 wt%), hydrogen tetrachloroaurate(III) hydrate (HAuCl4, 99.9 + %), potassium carbonate (K2CO3, 99.7 %), absolute ethanol (C2H5OH, 99.5 %), and sodium hydroxide (NaOH, semiconductor grade).
Systhesis
Silica nanoparticles were prepared via hydrolysis and condensation of TEOS using the sol-gel process. An am- monium hydroxide solution was used as the catalyst to cause the formation of spherical silica particles [6]. To prepare gold colloids of 1∼3 nm diameter, THPC/NaOH reducing agents were mixed with 1.0 wt% aqueous tet- rachlroloaurate(III) trihydrate. Under alkaline conditions, THPC is a reducing agent powerful enough to reduce gold salts by the derivation of formaldehyde [7]. An excess of APTMS was then added to the solution of silica nanoparticles. Next, functionalized silica nanoparticles (0.5 mL) were added to the gold colloids for the deposi- tion of gold colloids onto the silica nanoparticles.
Gold-deposited silica (Au-APTMS/SiO2) was further re- duced by reacting with reducible gold salts to form the gold layer over the silica cores. The reducible gold salts were prepared through the addition of potassium carbo- nate to 1.0 wt% HAuCl4 solution; the color of the solution slowly changed from yellow to colourless.
Results and Discussion
Preparation of Colloidal Silica Spheres
Silica nanoparticles (90∼180 nm) prepared by Stöber method at 48 oC exhibited zeta-potentials of ca. -70∼ -80 mV; they were very stable during two-week storage at 4
oC. The size of the silica nanoparticles was controlled by the addition of 28 wt% ammonium hydroxide solution at a fixed reaction temperature. As shown in Figure 2, the part-
Figure 3. UV-Vis spectra of gold colloids prepared using THPC/
NaOH reducing agents plotted with respect to the aging time at 4
oC.
(a) (b)
Figure 4. TEM images of Au-APTMS/SiO2 samples prepared using (a) one-day-aged and (b) one-week-aged gold colloids.
icle size did not increase linearly because of various influ- encing factors, such as the molar ratio of [H2O]/[TEOS], the feeding rates, and the pH. The increased ammonium- ion-dosages led to the formation of larger silica nano- particles; high contents of residual water caused a rapid nucleation rate via hydrogen-bonding-induced agglomer- ation between silica nanoparticles [8]. In comparison to the results at room temperature, an elevated reaction tem- perature (48 oC) resulted in smaller particles with narrower size distributions due to enhanced nucleation rates (when other reaction conditions were kept constant) [9].
Aging Effects of Gold Colloids
THPC-induced gold colloids (1∼3 nm) exhibited zeta- potentials in the range of -40∼-45 mV. Figure 3 exhibits the variations of the UV-Vis spectra of gold colloids with various aging times. The absorption intensity increased slightly at ca. 520 nm with increasing aging times, indicat- ing a slight variation of colloidal stability and particle size [10]. The pH of one-week-aged gold colloids gradually decreased to pH 7∼8 from an initiall pH 10, possibly be- cause the residual OH- ions were gradually consumed by THPC reducing agents during the aging period at 4 oC.
Figure 5. UV-Vis spectra of Au-APTMS/SiO2 samples prepared using gold colloids of different aging times.
To attach the gold colloids onto the silica cores, the silica surface was self-assembled using APTMS with amino groups having positive zeta potentials. The attachment of gold colloids was achieved through electrostatic attraction between the aminated silica nanoparticles and the gold colloids having negative charges. Two types of gold col- loids (i.e., one-day-aged and one-week-aged gold colloids) were used in the preparation of the gold-deposited silica nanoparticles (Au-APTMS/SiO2). Figure 4 compares TEM images of the Au-APTMS/SiO2 samples prepared using the one-day-aged and one-week-aged gold colloids.
The sizes of gold seeds attached to the silica nanoparticles was roughly estimated from the TEM images. The one- week-aged gold colloids particle size: ca. 3.5 nm exhibited a monodisperse deposition of gold seeds on the silica nanoparticle surfaces. In contrast, the Au-APTMS/ SiO2
sample prepared using the one-day-aged gold colloids ex- hibited a relatively heterogeneous deposition of gold seeds, with a particle size of ca. 5∼10 nm that was much larger than the mean particle size of the THPC-induced gold colloids.
Gold-deposited silica samples exhibit UV-Vis spectra that reflect the characteristics of gold seeds attached to the silica nanoparticle surface. As shown in Figure 5, peaks in the plasmon-derived absorption spectra were positioned around 534 and 509 nm, respectively, for the one-day- aged and one-week-aged gold colloids. The differences in peak positions and absorption intensities were caused by the cluster sizes of the deposited gold seeds on the silica nanoparticles, i.e., the stronger plasmon resonance was caused by larger-sized gold clusters. However, it is im- portant to prepare monodisperse gold-deposited silica for the fabrication of strong NIR-absorbing gold nanoshells [11]. Hence, one-week-aged gold colloids were used in the following experiments to determine the growth proc- ess of the gold layers over the silica cores.
150 nm 80 µL 4 mL (1.52 mM) 30 nm 5 845 nm 0.89
150 nm 120 µL 4 mL (1.52 mM) 30 nm 5 825 nm 0.81
150 nm 80 µL 4 mL (1.52 mM) - - 856 nm 0.45
110 nm 26.8 µL 4 mL (1.14 mM) - - 699 nm 0.37
110 nm 40 µL 4 mL (1.14 mM) - - 810 nm 0.41
110 nm 53.6 µL 4 mL (1.14 mM) - - 810 nm 0.51
110 nm 53.6 µL 4 mL (1.52 mM) - - 810 nm 0.74
110 nm 67 µL 4 mL (1.52 mM) - - 810 nm 1.06
110 nm 80 µL 4 mL (1.52 mM) 28 nm 4 811 nm 1.09
a Absorbance difference between the heights of plasmon resonance bands.
* ‘-’ denotes unavailable (or immeasurable) experimental data resulting from the limited resolution of the SEM instrument and/or some agglomeration of gold nanoshells.
Figure 6. UV-Vis spectra of gold nanoshell recorded during a growth process of 5 min. The solid line is the initial UV-Vis spectrum of Au-APTMS/SiO2. Other lines indicate the evolution of the optical resonance as the gold layers grew over the silica cores.
Concentration Effects of Gold Reducible Salts
The gold seeds attached to the silica nanoparticles were further reduced by reacting with gold-reducible salts, con- sequently leading to continuous gold layers on the silica nanoparticle surface via coalescing of neighboring gold clusters. Figure 6 displays the evolution of the UV-Vis spectra during the growth process of the gold layers over
the silica cores within five minutes. The optical plasmon absorption of the gold nanoshells red-shifted strongly with the progressive coverage of the gold layer on the silica nanoparticle surface; the color of the solution changed from colorless to blue and then to dark-red during the five minute period.
The formation of gold layers on the silica cores was con- ducted at several concentrations of gold-reducible salts at optimal dosages of formaldehyde reducing agents. Figure 7 shows SEM images of the gold nanoshells prepared by the addition of gold salts (from 0.38 to 1.52 mM) at a fixed dosage of formaldehyde (1.07 mmol). When the available gold ion concentration was too low (in the case of 0.38 mM gold salts), the gold clusters formed on the silica surface were not fully grown to cover the whole sili- ca nanoparticle surface, as shown in Figure 7(a). When a two-fold concentration (0.76 mM) of gold salts was add- ed, the coverage of the gold layer increased, as shown in Figure 7(b). When the concentration of gold salts was in- creased to 3∼4 times of the initial 0.38 mM, the gold lay- er almost covered the silica nanoparticle surface, as shown in Figure 7(c), and the thickness of the gold layer in- creased correspondingly. The excessive addition of gold salts (more than 2.0 mM) and/or formaldehyde reducing agent (more than 1.6 mmol), however, led to the decrease of the plasmon-derived absorption intensity because of the
(a) (b)
(c) (d)
Figure 7. SEM images of gold nanoshells (110 nm silica core) prepared at different concentrations of gold salts: (a) 0.38 mM gold salts, 1.07 mmol HCHO; (b) 0.76 mM gold salts, 1.07 mmol HCHO; (c) 1.52 mM gold salts, 1.07 mmol HCHO; (d) 1.52 mM gold salts, 0.67 mmol HCHO.
Figure 8. UV-Vis spectra of gold nanoshells formed at (a) different concentrations of gold salts and (b) different dosages of reducing agents.
agglomeration between gold nanoparticles (data not shown). Figure 7(d) shows the SEM image of gold nano- shells formed at 1.52 mM gold salts and 0.67 mmol formaldehyde. In comparison to the gold nanoshells (shell
thickness = 30 nm) shown in Figure 7(c), the decrease of the reducing agents led to a smaller gold cluster and a thinner gold layer (shell thickness = 14 nm) on the silica nanoparticle surface.
In summary, the optical resonance of gold nanoshells was optimized by considering the relative contributions of the aging of gold colloids, the concentration of gold salts, and the dosage of reducing agents. To obtain strong NIR-absorbing nanoshells, it is important to prepare com- plete and thick gold layers onto the silica cores. Table 1 exhibits the optical characteristics of the gold nanoshells prepared under various conditions. The position of the ab- sorption peak of the gold nanoshells increased asymptoti- cally with the increasing core-shell ratio, which is in ac- cord with the theoretical predictions of Mie scattering re- ported by Halas and coworkers [13].
Conclusions
The morphologies of gold-deposited silica nanoparticles were determined by adjusting the aging times of THPC- induced gold colloids. One-week-aged gold colloids pro- duced a relatively monodisperse deposition of gold seeds onto the silica nanoparticles; in contrast, one-day-aged gold colloids produced a heterogeneous deposition of gold seeds on the silica nanoparticle surface. Gold nanoshells were fabricated through the monodisperse deposition of gold seeds onto silica nanoparticles by varying the con- centrations of reducible gold salts and reducing agents.
The gold nanoshells (core diameter = 150 nm; core/shell = 5) exhibited strong plasmon-derived absorption bands at ca. 810∼850 nm. Increasing the amount of reducible gold salts led to an increase of the gold layer thickness over the silica cores, accompanied by a distinct increase of the plasmon-derived optical resonance. Excessive amounts of
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