Direct Interaction Between Gold Nanorods and Glucose
© The Author(s) 2010
Received: 21 June 2010
Accepted: 1 July 2010
Published: 13 July 2010
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© The Author(s) 2010
Received: 21 June 2010
Accepted: 1 July 2010
Published: 13 July 2010
In this work, we present the results of the study on the interactions between gold nanorods (GNRs) and glucose. The optical properties of GNRs have higher sensitivity to glucose compared with that of gold nanospheres. The long-wavelength bands of the GNRs obviously decrease as the concentration of glucose increases. At high glucose concentrations, the absorption peak in long-wavelength bands almost disappears, and the absorption intensities corresponding to the transverse plasmon band are also decrease. These results suggest that glucose could seriously affect the optical properties of GNRs. A possible interaction mechanism between gold nanorods (GNRs) and glucose has been proposed. Furthermore, the influence of glucose on different amount GNRs also has been studied.
Nanometer-size noble metallic low-dimensional structures have received much attention in recent years due to their special electronic and optical properties [1–3]. Currently, there is a great deal of interest in metallic nanorods due to their shape-dependent optoelectronic properties. Among various 1-D metallic nanostructures, gold nanorods (GNRs) are of particular importance owing to their tunable surface plasmon resonance properties . In addition, because of the good biocompatibility and facile bioconjugation of gold, GNRs have been explored for biological and medical use as optical contrast agents for dark-field [5–9], two-photon luminescence diagnostic imaging  and photothermal therapy of cancer cells [11, 12].
GNRs exhibit more attractive optical properties when compared to gold nanospheres due to anisotropic shape. There are two major absorption bands in the electromagnetic spectrum of the nanorods: the wavelength maximum centered at ∼520 nm corresponds to the transverse plasmon oscillations of nanorods. It depends on the aspect ratio and the diameter of the nanorods. The second adsorption maximum around 650–800 nm is due to the longitudinal plasmon oscillations and possesses much stronger intensity and can be tuned by varying the length of the nanorods . The position and intensity of these bands can be affected by changes in the dielectric constant around the vicinity of these nanoparticles, known as localized surface plasmon resonance (LSPR) or nanoSPR [14–16]. These properties suggest that GNRs have several advantages for applications in biological sensing, imaging, and therapy, which may perhaps benefit from the geometry of these structures [17, 18]. Additionally, the elongated nanoparticles have an inherently higher sensitivity to the local dielectric environment compared to similarly sized spherical nanoparticles. More importantly, GNRs with different aspect ratios could be easily fabricated and their unique yet simple ‘‘multiplexing’’ advantage could be harnessed .
Successful using of GNRs in vivo at least requires engineering efforts to make them biocompatible and stable within in vivo microenvironment [20, 21]. However, the organism is a very complex system. There are many factors that will influence the optical properties of GNRs, and then has an effect on the use of GNRs. So it is crucial to investigate the interaction of GNRs with blood plasma and proteins [22, 23]. Blood sugar is one of the important basal components in blood plasma. As is well known, different people have different blood sugar levels, such as the normal amount of the glucose in human blood serum is 4–6 mM, and the glucose value of diabetes may more than 11 mM. It has been reported that glucose is used as a reducing agent in preparation of metal nanoparticles . So it could be assumed that glucose in blood serum would affect the optical properties of GNRs when they are used in the organism. To the best of our knowledge, no paper has reported the effect of glucose on the optical properties of GNRs.
In this paper, we investigated the absorption spectrums and stability changes of GNRs in the presence of glucose for the first time. The absorption of GNRs decreased on increment of glucose concentration. The absorption peak in long-wavelength bands gradually disappeared when the concentration of glucose was higher than 5.6 mM. The influences of glucose on GNRs with different aspect ratios or different amounts had also been studied. We think this study could make a basis of research and experimentation for the study of the application of GNRs in organism.
β-d Glucose and Cetyltrimethylammonium bromide (CTAB) were purchased from Sigma. l (+)-Ascorbic acid (99%), silver nitrate (AgNO3), chlorauric acid (HAuCl4·3H2O), and sodium borohydride (NaBH4) were products of Beijing Shiji Company. All the chemicals were used without further purification. All solutions were prepared with deionized water.
GNRs were prepared by a seed-mediated growth approach reported by Murphy et al. [25, 26]. First, the gold seeds were prepared. An aqueous solution containing 0.62 mM CTAB and 2.2 × 10−3 mM HAuCl4 was prepared in a conical flask. Next, 5 × 10−2 mM of ice-cold NaBH4 solution was added to the solution while stirring. The solution turned brownish yellow immediately after adding NaBH4, indicating particle formation. Next, the growth solution was prepared as follows: 0.6 mM AgNO3 was added to 0.95 mM CTAB solution. To this solution, 6.72 × 10−3 mM HAuCl4 was added, and after gentle mixing of the solution, 1 × 10−2 mM ascorbic acid was added. The final step was the addition of 0.02 mL of the seed solution to the growth solution. The temperature of the growth medium was kept constant at 25° in all the experiments. Then the precipitate was centrifuged, washed with deionized water, and diluted to 5 mL with deionized water. Gold nanospheres were prepared by the following steps: (1) An aqueous solution containing 0.95 mM CTAB and 6 × 10−4 mM AgNO3, 6.72 × 10−3 mM HAuCl4 was prepared in a conical flask; (2) 4.6 × 10−3 mM NaBH4 solution was added into the solution. The solution turned dark red immediately indicating particle formation, at last the precipitate was centrifuged, washed with deionized water, and diluted to 5 mL with deionized water.
Transmission electron microscope (TEM) of gold nanorods was obtained with a JEM-1230 electron microscope (JEOL, Japan), operating at 120 kV.
The UV–vis spectra of the samples were taken using a V-570 UV/VIS/NIR spectrophotometer (Jasco, Japan). Quartz cells of 1 cm optical path length were used for all spectrum measurements. The sample was prepared as follows: GNRs were dissolved into 2 mL by PBS buffer (pH = 6.8) with different concentration of glucose solution (concentration from 0.6 to 6.7 mM) for 2 min. Then, samples were contained in 1-cm path length quartz, and the UV–vis absorption intensities of the solution were recorded by JASCO V-570 UV/VIS/NIR spectrophotometer. The UV–vis absorption signal was recorded over a range from λ = 400 nm to λ = 800 nm.
Figure 1c shows the changes in the absorbance of the GNRs with larger aspect ratio interaction with different concentrations of glucose (R = 3.0). These nanorods have two absorption maxima at 523 and 696 nm corresponding to the transverse and longitudinal mode, respectively. The absorbances of the GNRs make the same changes with the concentration of glucose increasing. Figure 1d shows the calibration curve derived from the changes in the absorbance at λ = 696 nm as the concentration of glucose increase. The linear range scans the concentration of glucose from 0.6 to 6.7 mM with a correlation coefficient of 0.992.
The present study has demonstrated the direct interaction between GNRs and glucose. The UV–vis spectrum results show that the glucose has more significant influence on the longitudinal plasmon band of GNRs. As the concentration of glucose increase, the long-wavelength bands of the GNRs obviously decrease. This is probably due to the anisotropy of the GNRs. When the concentration of glucose is higher than 5.6 Mm, the absorption intensities corresponding to both the longitudinal plasmon band and the transverse plasmon band are greatly affected and the absorption peak in long-wavelength bands almost disappears. The experimental results prove that glucose has seriously affected the optical properties and stability of GNRs. This effect could be weakened by increasing the amount of GNRs. So this study would help the biomedical applications of GNRs.
Support for this research by grants from the National Natural Science Foundation of China (project No. 20873171, 60736001) and National Hi-Tech 863 Program (No. 2007AA021803) is gratefully acknowledged.
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