Size dependence of Au NP-enhanced surface plasmon resonance based on differential phase measurement

doi:10.1016/j.snb.2012.09.073

Sensors and Actuators B: Chemical

Volume 176, January 2013, Pages 1128-1133

https://doi.org/10.1016/j.snb.2012.09.073 Get rights and content

Abstract

The impact of spherical gold nanoparticles (Au NPs) with diameters of 40–80 nm for the enhancement of surface plasmon resonance (SPR) sensing signals is presented. Numerical analysis is given to simulate the perturbation of evanescent field in the presence of Au NPs. The results indicate that Au NPs with 40 nm possess the highest coupling effect when the separation of Au NP and SPR sensing film is fixed at 5 nm. For experimental demonstrations, colloidal Au NPs with different sizes but unified extinction coefficient (optical density) are immobilized onto SPR sensing films respectively through a spacer, dithiothreitol (DTT). Phase changes of the reflected SPR signals, which are associated with the plasmonic coupling between the NPs and sensing film, are monitored using a differential phase SPR sensor. Results obtained from the experiments show good agreement with the theoretical studies. This work can considerably serve as a solid guidance for future development of Au NPs-enhanced SPR sensors.

Introduction

Surface plasmon resonance (SPR) sensors have attracted great attention over the past decade for their powerful capability and flexibility of studying various chemical and biological interactions [1], [2], [3]. The sensing mechanism is based on the fact that very small refractive index change at the metal/dielectric interface can affect the surface plasmons excited by incident light, thereby optical characteristics (intensity, phase and polarization) of the reflected light are varied accordingly and can be monitored in real-time. In conventional SPR sensors, usually based on angle-dependent reflectance measurement, are limited for detecting low molecular weight molecules such as TNT, DNA, cytokine and hormones. So far, there are two main strategies for overcoming this drawback: (i) developing phase-sensitive SPR sensors, (ii) using nanoparticle-based detection regimes [4], [5], [6], [7], [8]. It is reported that by modifying the sensing film with colloidal spherical gold nanoparticles (Au NPs), gold nanocages (Au NCs) and gold nanorods (Au NRs), the sensitivity can be significantly enhanced [9], [10], [11]. Among these nanoparticles, spherical Au NPs are more favorable to the enhanced SPR sensors because of their isotropic structures which allow the coupling to occur at every direction rather than only in the longitudinal direction for Au NRs [12]. Strong absorption of incident light followed by field enhancement on the metallic surface (Au NP and Au thin film) is a typical property of SPR excitation [2]. The principle of Au NPs-enhanced SPR is due to the strong field coupling between localized SPR (LSPR) of Au NPs and SPR of the sensing film, thus it is important to unify the LSPR of Au NPs during the size dependence study of Au-enhanced SPR. LSPR of Au NPs is represented by their extinction coefficient (or called optical density) which is determined by the absorption and scattering efficiency [13]. For Au NPs with different sizes, the absorption and scattering efficiency contribute differently to the LSPR, and therefore will lead to different field coupling effects to Au NPs-enhanced SPR sensors. Furthermore, the sensitivity of Au-enhanced SPR sensors is demonstrated to be directly related to the excited electric field strength and the perturbation (dip angle shift or phase change) of SPR curves [6], [14]. However, not much effort has been given to address the size issue of Au-enhanced SPR sensors, especially for Au NPs > 40 nm. In this paper, first, we numerically investigate the correlation between evanescent field perturbations and different sizes of Au NPs (40 nm, 60 nm, 70 nm and 80 nm) with 5 nm apart from the sensor surface. Then we experimentally demonstrate the size dependence of Au NPs-enhanced SPR using differential-phase detection method. Using DTT as the spacer, colloidal Au NPs with diameters of 40 nm, 60 nm, 70 nm and 80 nm have been immobilized on the sensor surface for investigating their coupling effect to the enhanced SPR sensing signals.

Section snippets

Theoretical modeling

According to the conditions for surface plasmon excitation in the Kretchmann configuration [2], the wave vector of incident light projected in the x direction k_x, which is parallel to the sensing film, should be matched with the wave vector of the surface plasmon oscillations k_sp:

$k_{x} = k_{s p} where k_{x} = k_{0} n_{prism} \sin θ_{inc}$

Here k₀ is the wave vector of the incident light in free space, n_prism = 1.76552 is the refractive index of the prism at wavelength of 785 nm, θ_inc = 52.0° is the incident angle. Thus, the

Materials

dl-Dithiothreitol (BioUltra, ∼1 M in H₂O), glycerin (≥99%), formamide (BioReagent, ≥99.5%) were purchased from Sigma Aldrich. Spherical gold nanoparticles (OD = 1.0) with diameters 40 nm, 60 nm, 70 nm and 80 nm were purchased from Nanopartz Accurate. The size variance is less than 4%. All chemicals were used as received without further purification. Ultrapure deionized (DI) water was obtained by Spectra-Teknik water purification system. Glass slides coated with 50 nm of gold film were purchased from

Performance of the SPR sensing system

Fig. 3 shows the calibration results of our home-built SPR sensing system, different weight ratios (1%, 2.5% and 5%) of glycerin and formamide solutions were introduced into the sensor head, respectively. The corresponding refractive index from pure water to 5% glycerin is 1.333–1.339 [17]. For 1% glycerin, the refractive index change from water is 0.0012 refractive index units (RIU) and a phase change of 22.16° is obtained (see Fig. 3(a)). Stability of the system was also monitored for 10 min,

Conclusions

In our work, the size-dependent signal response in terms of phase change has been demonstrated in an Au NPs-enhanced SPR sensing system. The particle size used here ranges from 40 nm to 80 nm, which extinction coefficients are composed of two components: absorption and scattering. Results from differential-phase SPR measurements agree well with those obtained from theoretical modeling using finite element methods. It is revealed that the enhanced SPR sensing signals decrease with increasing

Acknowledgements

This work is supported by the Start-up Grant (M4080141.040) from Nanyang Technological University and Singapore to China Joint Grant (SERC 0921450031, China 2009DFA12640).

Shuwen Zeng is currently a Ph.D. student of the School of Electrical and Electronic Engineering at Nanyang Technological University.

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Shuwen Zeng is currently a Ph.D. student of the School of Electrical and Electronic Engineering at Nanyang Technological University.

Xia Yu received the B.Eng. and Ph.D. degrees in Electrical and Electronic Engineering both from Nanyang Technological University in 2003 and 2006 respectively. She is currently a Research Scientist with Precision Measurements Group, Singapore Institute of Manufacturing Technology (SIMTech). Her research interests include surface plasmonic sensing devices, photonic crystal fibers and microstructured waveguides.

Wing-Cheung Law received his Ph.D. from the Department of Electrical Engineering of the University at Buffalo, the State University of New York (SUNY) in 2011. He is now a research assistant professor of University at Buffalo – SUNY. His research interests include surface plasmon resonance biosensors and quantum dots for bioimaging.

Yating Zhang is a Ph.D. student of the College of Optoelectronic Science and Engineering at Huazhong University of Science & Technology. She is also currently an exchange research student at Singapore Institute of Manufacturing Technology (SIMTech).

Rui Hu received his Ph.D. from the Department of Optical Engineering of Zhejiang University in 2010. He is currently a research fellow at school of Electrical and Electronic Engineering, Nanyang Technological University.

Xuan-Quyen Dinh received his Ph.D. in Physics from Ecole Normale Supérieure (ENS) de Cachan, FRANCE in 2007. He is currently the deputy-director of CNRS International-NTU-Thales Research Alliance (CINTRA), Singapore. His research interests include fiber-optic communications, micro-nano fibers for sensing applications and quantum key distribution.

Ho-Pui Ho received his B.Eng. and Ph.D. in Electrical and Electronic Engineering in the University of Nottingham in 1986 and 1990 respectively. He is now a professor at The Chinese University of Hong Kong. His research interests include optical instrumentation, photonic biosensors based on the surface plasmon resonance effect, biophotonics and materials for optical applications.

Ken-Tye Yong received his Ph.D. from Chemical and Biological Engineering in SUNY at Buffalo in 2006. Following completion of his graduate studies, he did his post-doc at the Institute for Lasers, Photonics and Biophotonics from 2006 to 2009. He is currently an Assistant Professor at the Nanyang Technological University in the School of Electrical and Electronic Engineering. His research interests include metallic nanoparticles for sensing applications and quantum dots for cancer therapy.

View full text

Size dependence of Au NP-enhanced surface plasmon resonance based on differential phase measurement

Abstract

Introduction

Section snippets

Theoretical modeling

Materials

Performance of the SPR sensing system

Conclusions

Acknowledgements

Optics Communication

Biosensors and Bioelectronics

Biosensors and Bioelectronics

Analytical Biochemistry

Biosensors and Bioelectronics

Introduction to Biophotonics

Surface Plasmons on Smooth and Rough Surfaces and on Gratings

Surface plasmon resonance sensors for detection of chemical and biological species

Chemical Reviews

Phase and amplitude sensitivities in surface plasmon resonance bio and chemical sensing

Optics Express

Sensitivity improved surface plasmon resonance biosensor for cancer biomarker detection based on plasmonic enhancement

ACS Nano

A review on functionalized gold nanoparticles for biosensing applications

Plasmonics

Ultrasensitive surface plasmon resonance detection of trinitrotoluene by a bis-aniline-cross-linked Au nanoparticles composite

Journal of the American Chemical Society