Enhanced Sensitivity of Surface Plasmon Resonance Phase-Interrogation Biosensor by Using Silver Nanoparticles
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- Peng, T., Lin, W., Chen, C. et al. Plasmonics (2011) 6: 29. doi:10.1007/s11468-010-9165-4
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It is demonstrated that the sensitivity of surface plasmon resonance phase-interrogation biosensor can be enhanced by using silver nanoparticles. Silver nanoparticles were fabricated on silver films by using thermal evaporation. Sizes of silver nanoparticles on silver thin film can be tuned by controlling the deposition parameters of thermal evaporation. By using surface plasmon resonance heterodyne interferometey to measure the phase difference between the p and s polarization of incident light, we have demonstrated that sensitivity of glucose detection down to the order of 10−8 refractive index units can be obtained.
KeywordsSurface plasmonsPhase measurementSilver nanoparticlesBiosensor
Since the accomplishment of various optical methods in the excitation of the surface plasmon resonance (SPR) at a metal–dielectric interface , it has been well known that such an excitation can be utilized to achieve sensing of various interfacial phenomena with ultrahigh sensitivity. These include, for example, chemical and biological sensing [2–9], film-thickness sensing , temperature sensing , and angular measurement . Recently, it has been demonstrated that this SPR monitoring technique for chemical and biological sensing can achieve very high sensitivity, down to the order of 10−8 [8, 9] refractive index units (RIU).
For the sensing application by using SPR, it is straightforward to adopt the attenuated total reflection method by coupling incident light to excite surface plasmon resonance at the metal-dielectric interface, leading to a dip in the reflection spectrum which can be monitored to follow any property changes that take place in the proximity . There are at least four kinds of parameters that can be monitored in the SPR sensing process: (a) the change of the resonant angle, (b) the change of reflectance at fixed incident angle, (c) the change of resonant wavelength at fixed incident angle, and (d) the phase difference between p- and s-polarized light in the reflection spectrum . The detection methods that correspond to these four kinds of parameters are angular interrogation, intensity interrogation, wavelength interrogation, and phase interrogation, respectively.
Among these various monitoring techniques, it is well known that the “phase interrogation” technique is by far the more sensitive one in many applications [3, 5–9, 11, 12]. Recently, we have found that the sensitivity of this technique has strong dependence on the wavelength of incident light in our study of SPR temperature sensing . In other experiment, we have also observed that high-resolution angular measurement and biological sensing can be achieved by SPR phase interrogation at optimized incident wavelength [9, 12]. However, the value of the optimal wavelength will change when the film thickness varies. It is thus possible to reach optimal sensitivity by tuning the thickness of metallic film at fixed wavelength of incident light. Furthermore, modification of the surface roughness of the metallic film will also change the sensitivity of SPR sensor [1, 13, 14].
It was theoretically reported in the literature that metallic nanoparticles dispersed over metallic film could enhance the sensitivity of SPR sensor . This theoretical prediction has been experimentally demonstrated by immobilizing colloidal Au nanoparticles onto a thermally evaporated Au film. It was found that large changes in SPR reflectivity could be reached using this wet-chemistry approach . However, it is time consuming in the film production process where colloidal Au nanoparticles had to be first prepared via citrate reduction of HAuCl4 . If one can prepare the nanoparticle film in only one deposition process, this kind of SPR sensing chip will be more promising in practical applications.
Gold and silver island films on glass substrate were recently reported to be reproducible by precise control of the deposition parameters of thermal evaporation with tunable SPR wavelengths . Specific combinations of substrate temperature, deposition rate, and film thickness could produce island films with SPR wavelength tuning from visible to infrared. Based on this technique, one can employ thermal evaporation to produce metallic nano-thin film on the glass substrate first, and then control the deposition parameters to evaporate metallic nanoparticles over metallic film without opening the vacuum chamber. The size of nanoparticles can be controlled by deposition parameters, and therefore, the SPR wavelength of nanoparticle film could be tuned. As mentioned above, the wavelength of incident light is crucial to the sensitivity of SPR sensor based on phase-interrogation technique [9, 11, 12]. It is the purpose of our present work to extend SPR phase interrogation to refractive index measurement of glucose based on silver nanoparticle film produced by thermal evaporation. By controlling the deposition parameters of manufacture process, the size of silver nanoparticle can be changed, and, therefore, it is possible to achieve optimal sensitivity at fixed wavelength of incident light.
SPR sensing chips based on silver nanoparticles over silver thin film were fabricated by using thermal evaporation. The main parameters of the thermal evaporation include the substrate temperature (Ts), deposition rate (Rd), and deposition thickness of the thin film (Tf). Tf is determined from the quartz oscillator of thermal evaporator. A thin silver film was first evaporated on the glass substrate, and metallic nanoparticles were then evaporated over metallic film by controlling the deposition parameters. The thermal evaporation parameters for silver nanoparticles are Ts from 50 to 200 °C, Rd from 0.3 to 1.2 Å/s, and Tf from 10 to 100 Å, respectively. SPR chips were fabricated under different combination of these manufacture parameters. The sensitivity of SPR phase detection for glucose sensing based on these silver nanoparticle films were compared, and optimal fabrication condition was determined.
Fabrication parameters of silver nanoparticle films and detected SPR phase sensitivity by using these films
Substrate temperature (°C)
Deposition thickness of nanoparticles layer (nm)
Average size of nanoparticles (nm)
Size deviation of nanoparticles (nm)
Sensitivity of SPR phase interrogation (RIU)
1.4 × 10−6
1.1 × 10−6
4.6 × 10−7
5.1 × 10−7
5.8 × 10−7
8.2 × 10−7
5.3 × 10−7
6.3 × 10−8
3.8 × 10−7
4.6 × 10−7
The morphology of the nanostructures was analyzed using atomic force microscope (AFM, Nanosurf Mobile S). All of the AFM images were collected in the contact mode with an applied force of 18 nN. The nanoprobe tips were made of silicon with 10 nm in diameter, and the scanning range was 3 μm × 3 μm.
Results and Discussion
In the fabrication process of SPR sensing chip, we first used thermal evaporation to fabricate a 50-nm-thick silver thin film with substrate temperature of 100 °C and evaporation rate of 0.4 Å/S. Following this, we fabricated silver nanoparticles on the silver thin film with thermal evaporation rate of 0.4 Å/S, film thickness of 10 nm, and different temperature setting for different samples, including 50, 75, 100, 125, and 150°C. These samples were labeled as sample 1, sample 2, sample 3, sample 4, and sample 5, respectively. We then fixed substrate temperature to 100°C and tried to find the optimal parameter in film thickness by varying film-thickness parameter for different sample. The film thickness were 1, 3, 5, 7, and 10 nm and were labeled as sample 6, sample 7, sample 8, sample 9, and sample 10, respectively.
We have successfully demonstrated that SPR sensing chip based on silver nanoparticles over silver thin films could be fabricated by controlling the fabrication parameters of thermal evaporation; and the detection sensitivity of SPR phase interrogation for glucose detection could be enhanced by using this kind of sensing chip. With substrate temperature 100°C, deposition rate 0.4 Å/S, and film thickness 5 nm, we could fabricate silver nanoparticle film with the size of nanoparticles about 40 nm in diameter over Ag film of 50 nm in thickness. We have also demonstrated that the sensitivity of SPR phase interrogation for glucose detection based on this silver nanoparticle film could be as low as 6.3 × 10−8 at the laser wavelength of 632.8 nm. Unlike the traditional approach for enhanced SPR sensing using nanoparticles, which is time consuming in the film production process, where colloidal Au nanoparticles were first prepared via citrate reduction of HAuCl4 , our silver nanoparticle film could be prepared in only one deposition process. We believe that this kind of SPR sensing chip combined with SPR phase-interrogation technique will provide more promising applications and simpler protocols.
H.-P. Chiang acknowledges the financial support from the Center for Marine Bioenvironment and Biotechnology, National Taiwan Ocean University, and the National Science Council of ROC under grant number NSC 97-2112-M-019-001-MY3.