Hybridization of localized surface plasmon resonance-based Au–Ag nanoparticles
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- Zhu, S. & Fu, Y. Biomed Microdevices (2009) 11: 579. doi:10.1007/s10544-008-9267-3
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The hybrid Au–Ag triangular nanoparticles were proposed for the purpose of biosensing. To construct the nanoparticles, an Au thin film was deposited on top of the Ag nanoparticles supported with glass substrate. The hybrid nanoparticles can prevent oxidation of the pure Ag nanoparticles due to the Au protective layer caped on the Ag nanoparticles. The hybrid nanoparticles were designed using finite-difference and time-domain algorithm. Extinction spectra of the hybrid nanoparticles excited by visible light beam with plane wave were calculated, and the corresponding electric fields at peak position of the extinction spectra were expressed also. It is clear that the hybrid nanoparticles can excite the localized surface plasmon resonance wave which can be used to detect biomolecules. As an application example, we presented relevant detection results by means of using protein A to covalently link surface of the hybrid nanoparticles. Refractive index sensitivity of the hybrid nanoparticles was derived through both computational numerical calculation and experimental detection. Both the calculated and the experimental extinction spectra show that the hybrid Au–Ag nanoparticles are useful for detecting the biomolecules.
KeywordsHybrid Au–Ag nanoparticlesNano-biosensorLSPRSpectroscopy
Localized surface plasmon resonance (LSPR) nanobiosensors are of great interest in various applications such as environmental protection, (Ji et al. 2004; Ligler et al. 2003) bionanotechnology, (Kohls and Scheper 2000) and food safety (Ligler et al. 2003, Wiskur and Anslyn 2001). Recently, several research groups have begun to explore alternative strategies for development of the optical biosensors and chemosensors (Malinsky et al. 2001) working on the basis of the extraordinary optical properties of noble metal nanoparticles. They possesses a strong ultraviolet-visible absorption band that is not presented in the spectrum of the bulk metal. (Haynes and Van Duyne 2001) This absorption occurs when the incident photon frequency is resonant with the collective oscillation of the conduction electrons and is know as LSPR. It is well established that the nanoscale chemosensors and biosensors can be realized through shifts in the LSPR-based resonance peak transmission λmax of extinction spectrum of the silver nanoparticles. (Malinsky et al. 2001) These wavelength shifts are caused by adsorbate-induced local refractive index variations in competition with charge-transfer interactions on the nanoparticles surface. In this research domain, representative works have been performed by Northwestern University. (Haes and Van Duyne 2002; Haes and Van Duyne 2003; Riboh et al. 2003; Haes et al. 2004, Yonzon et al. 2004) One of their research subjects focused on the measurement of binding signal between antigen and antibody with the Ag triangular nanoparticles. However, there are high requirements for researching novel LSPR-based hybrid metal nanostructures. In this paper, considering microfabrication capability and the application request for the nanoparticles of the biochemistry chip, a hybrid Au–Ag triangular nanoparticles is introduced for achieving higher sensitivity. Using finite-difference and time-domain (FDTD) algorithm, we designed the extinction spectra and the corresponding electric fields at the extinction spectra peak position of the hybrid nanoparticles. We developed an extended nanosphere lithography (NSL) method to fabricate the hybrid Au–Ag triangular nanoparticles array with in-plane widths of ∼100 nm, out-of-plane heights of Ag ∼50 nm, upper Au thickness of ∼5 nm, and 400 nm period of the particles array.
2 Hybrid Au–Ag nanoparticles
3 Experimental setup
3.1 Chemical materials
Our experiments were carried out using Protein A which was from Sigma-Aldrich; and phosphate buffer solution (PBS, 0.01 M, pH 7.4)) from Jinshan Chemical Analyte Pte. Ltd. The buffer used in the experiments was prepared using double glass-distilled water.
3.2 Integrated LSPR sensor
4 Fabrication of the LSPR-based nano-biosensor
5 Results and discussion
We proposed a hybrid Au–Ag triangular nanoparticles array for the purpose of detection of a protein A. The hybrid Au–Ag particles can protect oxidation and sulfuration of the pure Ag particles from ambient environment. The typical protein can directly bind on the surface of Au film. Using our developed LSPR-based nano-biosensor with the hybrid Au–Ag nanoparticles, we can realize the refractive index sensitivity of 200 nm/RIU at atmosphere. The nano-biosensor demonstrates the potential applications in monitoring, detection and identification of biological agents, immunoassay as well as characterization of intermolecular interactions.