Review of Surface Plasmon Resonance and Localized Surface Plasmon Resonance Sensor

minghai@ustc.edu.cn Abstract: An overview of recent researches of surface plasmon resonance (SPR) sensing technology in Laboratory of Science and Technology of Micro-Nano Optics (LMNO), University of Science and Technology of China, is presented. Some novel SPR sensors, such as sensors based on metallic grating, metal-insulator-metal (MIM) nanoring and optical fiber, are designed or fabricated and tested. The sensor based on localized surface plasmon resonance (LSPR) of metallic nanoparticles is also be summarized. Because of the coupling of propagating surface plasmons and localized surface plasmons, the localized electromagnetic field is extremely enhanced, which is applied to surface-enhanced Raman scattering (SERS) and fluorenscence enhancement. Future prospects of SPR and/or LSPR sensing developments and applications are also


Introduction
Since the first application of surface plasmon resonance (SPR) phenomenon for gas detection and biologic sensor in 1982 [1], the SPR sensing technology has been widely used for the detection of biological and chemical analytes, environmental monitoring and medical diagnostics [2][3][4][5] in the past two decades.
Surface plasmons (SPs) are coherent oscillations of free electrons at the boundaries between metal and dielectric which are often categorized into two classes: propagating surface plasmons (PSPs) and localized surface plasmons (LSPs) [6]. PSPs can be excited on the metallic films which have several approaches as the Kretschman [7] and Otto [8] prism coupler, optical waveguides coupler [9], diffraction gratings [10], and optical fiber coupler [11], whereas LSPs can be excited on metallic nano-particles, which both can induce a strong enhancement of electromagnetic filed in the near-field region (resonance amplification), leading to a extensive application in surface-enhanced Raman scattering (SERS) [12], fluorescence enhancement [13], refractive index (RI) measurement [14], biomolecular interaction detection [15], and so on.
In this paper, we will review some recent works on SPR (based on PSPs) and localized surface plasmon resonance (LSPR) (based on LSPs) sensors at the Laboratory of Science and Technology of Micro-Nano Optics (LMNO) and make a prospect on the research and applications of SPR and LSPR sensors.

Current research activities on SPR and LSPR sensors at LMNO
Our research works are focused on new-style SPR sensors, bimetallic sensor chip for Kretschmann configuration, optical fiber SPR sensors and LSPR sensors on SERS and fluorescence enhancement.

Novel-style SPR sensors
Two types of SPR sensors with different styles are designed: metallic grating SPR sensor [16] and racetrack resonator SPR sensor [17]. Metallic grating SPR sensor has a high sensitivity for gas detection, and racetrack resonator SPR sensor has a broad linear detection range of analyte RI and high extinction ratio.

Metallic grating SPR sensor
We designed a highly sensitive grating-based SPR sensor for the gas detection [16]. The sensor has a high sensitivity at larger resonant incident angle if negative diffraction order of metallic grating is used to excite the surface plasmons, as shows in Fig. 1.
where m  is the permittivity of the metal, θ res is the resonant angle of incidence, n a is the RI of analyte, m is an integer representing the diffraction order, and sign "+" and sign "-" correspond to m﹥0 and m﹤0, respectively. The sensitivity (d res /dn) of the resonant angle of grating-based SPR sensors depends on the resonant incident angle (Fig. 2). The sensitivity of the negative diffraction order(m﹤0) is tens of times higher than that of positive diffraction order (m﹥0) at large resonant angle, which is also much higher than that of conventional prism-based SPR sensor. For hydrogen detection, a thin Pd film is deposited on the metallic grating. When the Pd-coated gold grating is exposed to hydrogen with different concentrations, the permittivity of Pd layer will change. Then the change in the resonant angle can be detected. The theoretical resolution of hydrogen concentration of the order of 0.001% is obtained according to our design.  Fig. 3 Reflectance spectra of rectangle-grating-based SPR sensors with different analyte refractive indices [18].
The sensitivity of the metallic-grating-based SPR sensor can be improved by using double-dips method [18]. As shown in Fig. 3, when the RI of the analyte changes, the resonant angle of negative diffraction order will change oppositely to that of positive diffraction order. The separation of negative diffraction order and positive diffraction order is detected to improve the sensitivity of metallic grating SPR sensors.

SPR sensor based on racetrack resonator
SPR sensing technique can be performed with an ultracompact racetrack resonator based on single mode metal-insulator-metal (MIM) plasmonic gap waveguide (Fig. 4).
Lc t E i2 Fig. 4 Schematic of an MIM plasmonic racetrack resonator coupled to a straight waveguide [17].
The sensor is a compact and integratable one which consists an MIM racetrack resonator and a straight MIM waveguide as an evanescent wave coupler. A broad-banded light propagates along the straight MIM waveguide, and the light satisfying the match condition will be coupled into the surface plasmon mode of the MIM racetrack resonator. The transmitted spectrum detected at the other end of the straight MIM waveguide gives a harp dip at the resonant wavelength, and the resonant wavelength will red-shift as the RI of ambient material (the insulator in a racetrack resonator) increases (Fig. 5).
The theoretical sensitivity ( / s n    ) of the racetrack resonator SPR sensor in this design is about 1030 nm/RIU. To achieve high coupling efficiency, the sensor should have a narrow resonant dip with a full width at half maximum (FWHM) of 9 nm and a high extinction ratio of 34.5 dB. The "figure of merit" (FOM) [19] is defined as follows: The FOM of the racetrack resonator SPR sensors can be up to 114.4 RIU -1 .

Traditional SPR sensors
The traditional SPR sensor contains two different parts: Krestchmann prism coupler and optical fiber coupler. Firstly, we investigated the temperature effect on the SPR sensor of Krestchmann configuration [20]; then, a bimetallic SPR sensor chip is designed and tested in Biacore3000 for biomolecular detection [21]; after that, wavelength-modulated and intensity-modulated optical fiber SPR sensors are studied, respectively [22,23].

Temperature effect on the SPR sensor of Krestchmann configuration
For actual applications of SPR sensor, ambient temperature will affect the measuring result [24]. As the most popular configuration, the effects of the temperature on an SPR sensor of Kretschmann configuration are investigated theoretically and experimentally.
Because of the change in properties of dielectric, metal, and analyte layers, the resonant dips are shifted and broadened at high temperature ( Fig. 6), which can be understood from the thermo-optic effect in the dielectric and analyte, together with a decrease of the plasmon frequency and an increase of the collision frequency in the metal layer when the temperature increases. The effects of the temperature from the sensor and the analyte are studied separately. The result shows that the thermal effect from the SPR sensor itself is much smaller than that of the aqueous analyte but comparable for the gas detection.

Bimetallic SPR sensor chip
Compared with traditional single gold films, the gold/silver bimetallic film has sharper SPR dip and higher sensitivity [25][26][27]. We deployed the bimetallic film into a commercial SPR instrument, Biacore3000 from GE Healthcare, to detect sulfamethoxazole (SMX), a kind of contaminations in food products, which is considered as a potential threat to consumers' health [28].
For the bimetallic sensor chip, a 40-nm silver film is deposited on the BK7 optical glass substrates, then, a 10-nm Au film is deposited on the 40-nm Ag film, which has the best FOM based on theoretical analysis. Figure 8 presents the atomic force microscope (AFM) images and resonant dips of the gold film and the gold/silver bimetallic film, respectively. The surface roughness values of the single Au film and Au/Ag bimetallic film (10-nm Au with 40-nm Ag) are 0.810 nm and 2.635 nm, respectively. The roughness values of the Au film and bimetallic film are of the same order and are smooth enough for the SPR sensing experiments. The FWHM of the bimetallic layer is about 2/3 times narrower than that of the single gold layer. For actual testing, the analysis cycle contains of the binding of the anti-SMX with the SMX on the bimetallic chip and the regeneration of the bimetallic chip surface. The real-time monitoring of the sensing progress and the stability of the sensor are measured for 6 times, as shown in Fig. 9. The limit of the detection of the SMX for the bimetallic film is 1.8 ng/μl, which is lower than that of the single gold film (3.2 ng/μl) [29] by comparing to traditional single gold films.

Optical fiber SPR sensors
Optical fiber SPR sensors have been studied extensively based on their unique advantages such as miniaturization, high spatial resolution, and capability of on-line distributed measurement and remote sensing in dangerous environments [30,31]. We designed and fabricated a novel wavelength-modulated optical fiber SPR sensor by using an ultra-thin dielectric film of MgF 2 as the modulated and protected layer, the influences of the dielectric film on the sensitivity and shape of the resonant dip are investigated which are significant to the SPR fiber sensor.
To fabricate the optical fiber SPR sensor, 40-nm silver and 10-nm MgF 2  As shown in Fig. 11, the transmittance spectra shift to a longer wavelength when the analyte RI increases. When the analyte RI is tuned from 1.3419 to 1.3730, the resonance wavelength varies from 552.7 nm to 631.2 nm. The resonance peaks broaden at the high RI because of the higher absorption loss at the longer wavelength.   Figure 12 presents the simulated and experimental results of the SPR spectrum change with different analyte refractive indices for the sensor with and without the MgF 2 film. When the liquid RI is lower (higher) than 1.38 (the RI of MgF 2 ), the sensor with the MgF 2 film has a longer(lower) resonance wavelength compared with the traditional structure (without MgF 2 ) because the MgF 2 film performs as a dielectric film that decreases (increases) the equivalent RI in the evanescent field. This is the modulation function of the dielectric film. Meanwhile, the sensitivity of the MgF 2 -protected fiber-optic SPR sensors increases monotonically with the analyte RI, which can reach 4464 nm/RIU when the RI is 1.3945. For a similar structure but without the MgF 2 film, the intensity-modulated optical fiber SPR sensor is studied with the radially polarized beam. Because of the rotational symmetry of the fiber, a radially polarized beam can improve the excite efficiency. The result is shown in Fig. 13. The sensitivity of the structure with the radially polarized beam is 3 times higher than that of using linearly polarized beam.

LSPR sensors
Due to the localized surface plasmon resonance, the local electromagnetic field around the metallic nanoparitcles will be enhanced dramatically, and the resonant wavelength is sensitive to the variation of the local dielectric environment. Our researches in this area include LSPR for the RI measurement, LSPs in the optical fiber for SERS enhancement [32][33][34], coupling between LSPs/LSPs and LSPs/SPs for SERS enhancement and fluorescence enhancement [35][36][37].

LSPR for RI measurement
Noble metal nanoparticles，such as gold and silver, exhibit a strong optical extinction at visible and near-infrared (NIR) wavelengths and generate LSPR phenomenon which is sensitive to the surrounding medium. Gold nanorod is widely used for refractive index sensing because its longitudinal localized surface plasmon resonance is very sensitive to the ambient refractive index. For the experiment, gold nanorods are easily be fabricated by the seeded growth method [38], and its aspect ratio can also be tuned to have the proper resonant wavelength (Fig. 14). Figure 14 illustrates the extinction spectra of two kinds of gold nanorods with different aspect ratios of 2.5:1 and 4.2:1. The longitudinal LSPR peak shifts to the longer wavelength as the refractive index increases, and the shift amplitude depends on the aspect ratio. The longer the aspect ratio is, the larger the shift will be. Thus we plot the longitudinal LSPR peak versus refractive index in Fig. 15 that shows nanorods with 4 different aspect ratios. It reveals an almost linear relationship between longitudinal LSPR peak and refractive index ranging from 1.33 to 1.47 in our experiment. As the aspect ratio varies from 2.5 to 4.2, the sensitivity increases from 216 nm/RIU to 352 nm/RIU.

Optical fiber SERS sensors
Two different types of optical fiber are used in our experiment: polymer optical fiber (POF) and photonic crystal fiber (PCF). Compared with the silica fiber, POF shows a more flexible, effective and robust performance and better biocompatibility in the sensing [39][40][41] while PCF can provide very large internal surface area because of air-hole arrays for SERS action. A POF SERS sensor and a PCF SERS sensor are developed in our experiment as follows: To fabricate the POF SERS sensor, we used the etching method [42] to prepare the POF tip with a diameter of 2 μm-3 μm. Then several drops of gold nanorods colloids were placed directly on the POF tip and dried naturally. After that, the POF SERS probe was dipped into rhodamine 6G (R6G) solution (10 -9 M) to detect the R6G SERS signal.
For the common "optrod" configuration [43], which the fiber is used to both transport the exciting laser radiation and collect the Raman scattering from the analyte, the Raman background of the POF would overwhelm the spectrum of the R6G dye. In this experiment, the light is focused on the tip directly with an inverted microscope objective, which is also used for collecting the signal. The spectra are achieved, as shown in Fig. 16. As a comparison, the same concentration analyte on the glass slide without gold nanorods is also measured. The SERS signal is enhanced 9-order magnitude which can be attributed to the local field of gold nanorods and their coupling. Additionally, a broad spectral PCF SERS sensor is developed by using the solid core holey PCF with the silver nanoparticle cluster. A PCF SERS sensor is fabricated by injecting the mixture of the 4-mercaptobenzoic acid (4-MBA) aqueous solution with the silver colloid solution into the air holes of the PCF.
The experiment setup is shown in Fig. 17. A continuous-wave Ar-ion laser (514.5 nm) and a diode laser (785 nm) are used as the excitation source to verify the broad spectral characteristics of the SERS probe. As shown in Fig. 18, by using the visible light of 514.5 nm and near-infrared light of 785 nm, the PCF SERS sensor has a good performance by obtaining an excellent and consistent SERS spectrum. Furthermore, the liquid holes' cladding results in a decrease of the numerical aperture and V parameter, which can be explained as that some high-order guide modes in air holes' cladding leak into the aqueous holes' cladding. Besides the low efficient way of the evanescent field, leakage modes can create important action in the solid core PCF SERS probe.

Coupling of LSPs in cube-array system for SERS enhancement
To enhance the intensity and localization of the electromagnetic field, the coupling between LSPs is investigated [44,45]. A quasi-three-dimensional system composed of silver nanocubes and silver nanohole arrays is fabricated (Fig. 19).
The silver nanohole arrays are thermally deposited on self-organized anodic aluminum oxide (AAO) substrates [46]. Then the R6G with a concentration of 5 × 10 -7 M in the polymethylmethacrylate (PMMA) solution is spin-coated on the Ag hole arrays to form a spacer layer. Afterward Ag cubes are chemically synthesized by reducing AgNO 3 with ethylene glycol [47][48] and dripped onto the R6G/PMMA layer. After drying at the room temperature for 24 hours, a coupled cube-hole array system is fabricated. AAO Fig. 19 Sketch of the coupled cube-hole array system [35]. Figure 20 presents the sketches of five types of structures and corresponding Raman spectra. Structure (d) is the coupled cube-hole array system with significant Raman signals. The enhancement factor (EF) [49] is of the order of 10 8 which is more than one order of magnitude larger than that of other structures due to the large field enhancement of the "hot spots" induced by the plasmonic coupling between the silver cubes and silver hole arrays.
To further understand the coupling effect, series of tests are conducted as the following works to investigate the influence of each geometrical parameter in structure (d) (Fig. 21).
Cube-hole array distance: The EF decreases exponentially with an increase of distance due to the electrical field coupling decaying.
Hole diameter: the EF drops down steadily with an increase of the hole diameter because of the reducing effective coupling area between the Ag hole arrays and Ag cubes.
Inter-hole spacing: the EF decrease steadily with an increase of the inter-hole spacing because the closer the holes are, the more interaction between the LSPs is, which produce larger field enhancement.
Silver film thickness: the EF increases with a decrease of Ag film thickness. Because the thin film can enhance the interaction of the LSPs between the two sides which raise the local field intensity in the whole coupling region.

Coupling between LSPs and SPs for SERS enhancement and fluorescence enhancement
The coupling between LSPs and SPs can strongly modify the electromagnetic field distribution, so the plasmonic interaction between silver nanocubes and a silver ground plane is studied for SERS and fluorescence enhancement.
In SERS experiment, a 57-nm-thick Ag film is evaporated on a glass substrate. PMMA films with different thicknesses are spin-coated onto Ag films. The Ag nanocubes are synthesized as mentioned above. With the mixture of a R6G solution and Ag colloid, the dyes will be adsorbed onto the cubes. Then the solution is dropped onto the PMMA film which forms the composite SERS substrate (S1) [ By using this novel substrate, the Raman signal can be detected from R6G molecules with a solution concentration down to 10 -11 M.
In the fluorescence enhancement experiment, the structure is similar to the structure mentioned above with only a little difference. The Rhodamine B (RhB) molecules are dropped in the PMMA as a spacer between the silver nanocubes and the silver film. The structure with no silver film is tested to make a comparison. Figure 23 presents the fluorescence spectra from RhB-doped PMMA films for different structures. The experimental results show that the enhancement factor on the Ag film substrate can reach 521, which is much larger than that on the glass substrate.

Prospects of research on SPR and LSPR sensors
As mentioned above, SPR and LSPR sensing technologies have a great application potentiality in many important areas including medicine, environmental monitoring, biotechnology, drug and food monitoring. But except for commercialized BIAcore and Texas Instrument sensors for biological sensing, other SPR and/LSPR sensing technologies still have not been deeply applied. To further overcome the limitation of the experimental condition and realize the extensive application, SPR/LSPR sensors have to be developed and improved to achieve more reliable requirements such as low cost, high sensitivity, facility, and robustness. Some future research works about SPR/LSPR can be expected in the following aspects: (1) The investigation and realization of the novel-style SPR senor with the high sensitivity and high accuracy in the laboratory and practical application.
(2) The effect of the humidity, pressure, electric/magnetic field and other factors which would affect the performance of the SPR/LSPR sensor in actual applications.
(3) The investigation on the multi-channel optical fiber SPR sensor which can enhance the sensing efficiency and accuracy.
(4) The application and commercialization of the LSPR sensor based on SERS and fluorescence enhancement for the biological/chemical detection.

Conclusions
In summary, an overview on research of SPR and LSPR sensors in LMNO is presented in this paper. Novel style SPR sensors include metallic grating SPR sensor and racetrack resonator. The SPR sensor is designed and developed which demonstrate its unique advantages such as high sensitivity, miniaturization, and broad linear RI detection range. To improve the sensing performance of the traditional SPR sensor, temperature effect, we investigated both bimetallic sensor chip and dielectric film coated optical fiber SPR sensor numerically and experimentally. Furthermore, the LSPR sensing technology is studied and developed. The SERS and fluorescence enhancement of the LSPR shows its attractive potentiality for biological and chemical applications.
Over the past two decades, SPR and LSPR sensing technologies have being widely investigated, and a large number of SPR/LSPR sensor platforms, commercialized SPR biosensor, and measurement formats have been developed. The research works that are reviewed in this paper will directly contribute to the development and application of the SPR/LSPR sensing technologies.