ReviewGold nanorod-based localized surface plasmon resonance biosensors: A review
Introduction
Biosensors have been continuing to play an important role in many scientific fields including clinical diagnostics, medical developments, illicit drug detection, food quality and safety, and environmental monitoring [1]. Their importance is seen in that each is a multi-billion dollar areas of activity and vitally important for the well-being of people the world over. A biosensor can be defined as an analytical device which comprises two basic components: a recognition unit used to capture the specific target, and a transducer that can convert the subtle biomolecular interactions into the quantifiable signal [2], which is usually electrical in nature. However, recent developments have seen an enormous growth in biosensors based on optical transducer principles, such as those using techniques such as fluorescence [3], [4], [5], surface plasmon resonance (SPR) [6], [7], [8] and chemiluminescence [9], [10], which have been developed for a wide range of applications. Among the various optical biosensors reported in the literature, SPR-based biosensors show a number of significant advantages over conventional sensors, including ultra-high refractive index sensitivity, fast sensor response, real-time detection, and a label-free technique. In addition, the advanced nature of SPR imaging (SPRi) technology not only retains these advantages of classical SPR sensing, but also allows the detection of target molecules on a biosensor chip to be visualized in real-time by using a CCD camera [11]. Thus SPR biosensors have been studied extensively and developed and commercialized in the past three decades in particular. SPR is an optical phenomenon where the surface conductive electrons of bulk metal oscillate collectively at their resonant frequency, and the electron oscillations propagate along the metal-dielectric interface and decay exponentially into both media [12]. However, in order to fabricate a SPR sensor chip, sophisticated instrumentation such as a sputter coater or vacuum evaporator is normally required to coat the noble metal film on the surface of an optical substrate, such as a prism and an optical fiber, to excite SPR. In addition, most commercial SPR instruments, such as the well-known Biacore™ series, are normally expensive and bulky, which limits the extent of their applications.
In recent years, biosensors based on localized surface plasmon resonance (LSPR), which is also a SPR phenomenon but exists in metallic nanoparticles (MNPs) rather than bulk metal, has attracted more and more attention. The physical properties of the noble metals change enormously from what is usually familiar when the size of these metal particles is on the nanoscale level and smaller than the wavelength of the light used to illuminate them [13], [14]. A particularly striking example is that the color of gold in the nanoworld is no longer the familiar ‘gold color’ but it can be as colorful as a rainbow, as shown in Fig. 1. Here gold nanorods (GNRs), with various aspect ratios and suspended in aqueous solutions, display a range of different colors. The SPR phenomenon also changed from SPR to LSPR when the bulk metal film was replaced by MNPs to excite SPR. Here the properties of LSPR are highly dependent on the material used and the size and shape of the metallic nanoparticles involved [14]. By manipulating these parameters, the LSPR wavelength can conveniently be tuned throughout the visible, near-infrared, and into the infrared region, allowing the LSPR sensor to be constructed for particular applications where a specific wavelength is desired. Compared to SPR sensors, LSPR sensors are of more flexible design and lower cost in terms of sensor fabrication, arising from the fact that LSPR can be excited when the light directly interacts with the MNPs and free from the need for prisms or other optical components. For instance, LSPR sensors can either be fabricated by immobilizing MNPs on a substrate, such as glass slide [15] or an optical fiber [16], or by simply suspending MNPs in solution to form a solution-phase based LSPR sensor [17]. In addition, as LSPR is highly localized at each individual MNPs, LSPR sensors can even be fabricated based on single nanoparticle [18]. Moreover, some LSPR biosensors have demonstrated superior sensitivity in comparison with the traditional bulk metal film based SPR biosensors [19], making them particularly attractive to use. These advantages of LSPR biosensors have prompted significant effort to be devoted to the development of sensitive LSPR biosensors and numerous promising LSPR biosensor designs continue to be reported in the literature, as this review emphasizes.
The advances seen in the fabrication of MNPs have led to considerable progress in the development of a range of LSPR biosensors in the past decade. Early research on the development of LSPR biosensor had mainly been focused on the use of spherical gold nanoparticles, due to their ease of synthesis. However recent developments have allowed a number of LSPR sensors, based on noble MNPs and of various shapes, to be developed and these have shown both higher sensitivities and other important advantages, in comparison to using gold nanosphere-based (GNS) LSPR sensors. Among these MNPs recently reported, GNRs have demonstrated unique optical properties, such as higher refractive index sensitivity and a tuneable longitudinal plasmon band, achieved by adjusting their aspect ratio [20], [21] and thus allowing them to show excellent characteristics as LSPR biosensors. In addition to LSPR sensing, GNRs have also been applied in many other fields such as SERS sensing [22], chemical imaging [23] and in cancer therapy [24].
Despite the fact that several excellent and well cited reviews on the general LSPR biosensors have been reported previously, these past reviews have not focused particularly on LSPR biosensors based on GNRs, to the best of our knowledge. This aspect is addressed directly in this paper which is designed to supplement the body of knowledge in this area by reviewing both key fundamental aspects and recent progress on the development of GNR-based LSPR biosensors. The paper thus deals with an overview of the underpinning sensing principles, the synthesis of GNRs, the surface modification of GNRs, the fabrication of a number of different biosensors and a range of biosensing applications, followed by a discussion of advances in GNR-based LSPR sensing. The paper ends with a view of future potential directions in research in this field.
Section snippets
Principles of LSPR
MNPs have particular optical properties which are significantly different from those observed in the bulk metal. When incident light interacts with MNPs, the electromagnetic field of the light induces a collective coherent oscillation of the surface conduction electrons of the MNPs in resonance with the frequency of light, in a phenomenon known as LSPR [14], [25]. The electric field of the light interacts with the free electrons in the nanoparticles, leading to a charge separation between the
Synthesis of GNRs
The successful development of LSPR sensors using GNRs depends on the reliable and accurate synthesis of GNRs that can then be applied to create consistent and reproducible sensors. The history of synthesis of spherical gold nanoparticles dates back more than a century. The most commonly used method for producing GNSs is citrate reduction, where the GNSs are synthesized by the addition into the boiling gold salt (HAuCl4) solution of a known amount of citrate solution, allowing the size of GNSs
Surface modification of the CTAB-capped GNRs
For GNRs synthesized in the presence of a CTAB surfactant using the wet-chemical methods, such as the seed-mediated growth method and electrochemical method, the surface of these GNRs is covered by a bilayer of positively charged CTAB molecules, as illustrated in Fig. 12. The CTAB surfactant is important to the synthesis of GNRs, because it not only works as a “structure-directing agent” to control the final particle shape, but also acts as a stabilizer to protect the as-synthesized GNRs
GNR-based LSPR biosensors
As LSPR can be excited when light directly interacts with metallic nanoparticles, the design of LSPR sensors is more flexible than that of SPR sensors. Both the GNS-based and the GNR-based LSPR sensors can share the same sensor configuration, in which the sensors can be configured by either immobilizing gold nanoparticles on a transparent substrate such as a glass slide or just simply leaving functionalized nanoparticles suspended in the solution in a cuvette where the detection will take place
Advances in LSPR biosensing
Advances in two specific fields, multiplexing of biosensors and single-nanoparticle based LSPR biosensing are highlighted below.
Conclusion and future outlook
In this review it can be seen that compared to other metallic nanoparticles, GNRs have shown both exceptional and highly desirable optical properties in LSPR-based biosensing applications, taking advantage of the high refractive index sensitivity and the potential for multiplexed sensing, all of which have attracted considerable attention from researchers focused on the development of GNR-based sensitive LSPR biosensors. Nevertheless, important challenges still remain in the practical
Acknowledgements
The authors would like to thank the Engineering and Physical Sciences Research Council (EPSRC) in the UK for the funding support via various schemes. The support of the George Daniels Educational Trust is also greatly appreciated.
Jie Cao received the B.E. degree in Electrical and Electronic Engineering from Harbin University of Science and Technology, Harbin, China, in 2004 and the M.E. degree in Mechatronic Engineering from Harbin Institute of Technology, Harbin, China, in 2007. He recently received the Ph.D. degree in Measurement and Instrumentation from City University London, London, UK. His research is focused on the design and fabrication of the SPR and LSPR based optical fiber sensors, and their biosensing
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Cited by (0)
Jie Cao received the B.E. degree in Electrical and Electronic Engineering from Harbin University of Science and Technology, Harbin, China, in 2004 and the M.E. degree in Mechatronic Engineering from Harbin Institute of Technology, Harbin, China, in 2007. He recently received the Ph.D. degree in Measurement and Instrumentation from City University London, London, UK. His research is focused on the design and fabrication of the SPR and LSPR based optical fiber sensors, and their biosensing applications.
Tong Sun received the received the B.E., M.E., and Dr. Eng. degrees in mechanical engineering from the Department of Precision Instrumentation, Harbin Institute of Technology, Harbin, China, in 1990, 1993, and 1998, respectively, and the Ph.D. degree in applied physics from the City University London, London, UK, in 1999. She is a Professor at the City University London, a member of the Institute of Physics and of the Institution of Engineering and Technology, and a Chartered Physicist and a Chartered Engineer in the UK.
Kenneth T.V. Grattan received the B.Sc. degree in physics from the Queen's University Belfast, Belfast, UK, in 1974, the Ph.D. degree in laser physics in 1979, and the D.Sc. degree from the City University London, London, UK, in 1992. He is a Professor at City University London, and the Dean of the City Graduate School, having formerly been Dean of the Schools of Engineering and Mathematical and Informatics. Prof. Grattan is a member of the Editorial Board of several major journals. He was awarded the Calendar Medal of the Institute of Measurement and Control in 1992, and the Honeywell Prize for work published in the Institute's journal as well as the Hartley Medal in 2012. He is a Fellow of the Royal Academy of Engineering, the UK National Academy for the field.