Plasmonics DOI 10.1007/s11468-013-9662-3
Surface Plasmon Resonance Imaging Sensors: A Review Chi Lok Wong & Malini Olivo
Received: 2 December 2013 / Accepted: 29 December 2013 # Springer Science+Business Media New York 2014
Abstract Surface plasmon resonance (SPR) imaging sensors realize label-free, real-time, highly sensitive, quantitative, high-throughput biological interaction monitoring and the binding profiles from multi-analytes further provide the binding kinetic parameters between different biomolecules. In the past two decades, SPR imaging sensors found rapid increasing applications in fundamental biological studies, medical diagnostics, drug discovery, food safety, precision measurement, and environmental monitoring. In this paper, we review the recent advances of SPR imaging sensor technology towards high-throughput multi-analyte screening. Finally, we describe our multiplex spectral-phase SPR imaging biosensor for high-throughput biosensing applications. Keywords Surface plasmon resonance imaging . Review . SPR . Non-labeling detection . Phase SPR imaging . Spectral SPR imaging
Introduction Surface plasmon microscopy was invented by Rothenhäuslar and Knoll in 1988 [1]. Plasmon surface polariton field was used to image microscopic interfacial structure. Since then, surface plasmon resonance (SPR) imaging has found rapid increasing research interests [2–5] and wide applications in
C. L. Wong (*) : M. Olivo (*) Bio-optical Imaging Group, Singapore Bioimaging Consortium, Helios #01-02, 11 Biopolis Way, Singapore 138667, Singapore e-mail:
[email protected] e-mail:
[email protected] M. Olivo School of Physics, National University of Ireland, Galway, Co., Galway, Ireland
drug discovery [6], biomarker screening [7], nucleic acid detection [5], food safety [8, 9], and environmental monitoring [10] in the past two decades. This paper reviews the recent advances in SPR imaging sensor technology. Excitation of Surface Plasmon Surface plasmon was observed in 1902 by Wood [11] with the photon-excited electrical resonance at small metallic particles. Plasmonic has gained wide research interests, including surface plasmon-enhanced Raman scattering (SERS) [12–18], localized SPR (LSPR) [19], surface plasmon field-enhanced fluorescence spectroscopy (SPFS) [20], plasmon-enhanced near-field scanning optical microscopy (plasmonic NSOM) [21], and SPR [1–5]. In 1968, Otto [22] reported the attenuated total reflection (ATR) coupling for the excitation of surface plasmon. In 1971, Kretschmann and Raether [23] presented the Kretschmann configuration ATR coupling, which is the most widely used excitation method in current SPR imaging sensors [1–5]. In ATR coupling, the excitation light passes through a high density medium (glass prism) and it modifies the phase velocity and wave vector of the excitation light (kin) [24, 25]. k in ¼
2π pffiffiffiffiffi ⋅sinθ εp λ
ð1Þ
where λ is the wavelength of the excitation light, θ is the incident angle, and εp is the dielectric constant of the prism material. The wave vector of the surface plasmon (ksp) propagating at the metal–dielectric interface is described by the following equation [24, 25], rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ωsp εm εs k sp ¼ ð2Þ c εm þ εs
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where ωsp is the angular frequency of the surface plasmon, εm is the dielectric constant of the metal film, and εs is the dielectric constant of the dielectric medium. The SPR phenomenon occurs when the energy of the excitation light (kin) matches that of the surface plasmon wave (ksp). At the resonance condition, part of the energy of the excitation light is transferred to the energy of surface plasmon. It produces an absorption profile in the reflection spectrum [3, 4, 25], which is known as the SPR absorption curve. Working Principle of SPR Imaging Intensity SPR Imaging SPR imaging sensors based on intensity [1, 26–31], angular [32, 33], wavelength [34–39], phase [40–45], and polarization [46–48] interrogations have been reported to date. Intensity SPR imaging has been widely used in different applications [4, 26–31] and commercialized by GWC Technologies (http://www. gwctechnologies.com). The operation principle is explained in Fig. 1a. Monochromatic light is used as the excitation source, and the incident angle θ is fixed at the resonance angle. Due to the energy transfer at the resonance condition, the intensity of the reflection light is attenuated and it produces an absorption profile in the reflection spectrum. According to Eqs. (1) and (2), the refractive index change of the dielectric medium varies the SPR absorption minimum wavelength, which varies the intensity of the reflection light (Fig. 1a). Intensity SPR imaging therefore provides a two-dimensional (2D) intensity contrast image of the refractive index distribution of the sensing surface [1, 26–31]. A typical intensity SPR image is shown in Fig. 1b. Spectral SPR Imaging According to Eq. (1), the wave vector of the excitation beam is wavelength dependent and it only matches with the propagation constant of the surface plasmon wave at particular wavelengths. In spectral SPR imaging, a polychromatic excitation source is used. The SPR absorption dip λdip shifts (Fig. 1a) according to the refractive index change of the dielectric medium, and it produces a corresponding color variation in the resultant image. Figure 1c shows a spectral SPR image reported by the authors [39] for an array of refractive index samples. Phase and Polarization Contrast-Based SPR Imaging In 1996, Nelson et al. [49] introduced phase detection for SPR sensing and a steep slope of phase response over a narrow range of refractive index was reported, which provides three times higher sensor resolution compared to angular and spectral SPR sensors [49]. As described by the Fresnel model [49, 50], the reflection coefficients (r) of p- and
s-polarizations can be expressed as, rp ¼ rp eiϕp and rs ¼ jrs jeiϕs
ð3Þ
where φp and φs are the phase of the p- and s-polarization, respectively. At surface plasmon resonance, the phase (φp) of the incident light is changed due to the energy transfer between light and surface plasmon [3, 25, 49]. Figure 1d shows a typical phase imaging in phase SPR imaging. However, the oscillation direction of the s-polarization light is perpendicular to the excitation plane of the surface plasmon wave and it is not affected at surface plasmon excitation. A phase difference (Δφ) is therefore produced between the p- and s-polarization light. It causes an orientation to the polarization ellipse, which becomes the operation principle of the polarization modulated SPR imaging sensors [46–48]. An SPR polarization contrast image is shown in Fig. 1e.
Intensity SPR Imaging Sensors Intensity SPR imaging was first demonstrated by Yeatman and Ash [51] and Rothenhäuslar and Knoll [1] in the late 1980s. In the past two decades, intensive research work on SPR imaging has been conducted by Corn et al. [26–29, 52–62]. In 1997, Jordan and Corn [26] used monochromatic He–Ne laser as excitation source where the expanded beam was made incident on the gold sensing surface at the resonance angle and the SPR image was subsequently captured by a monochromatic CCD camera. The system was used to characterize the electrostatic adsorption of proteins and synthetic polypeptides onto photopatterned monolayers of a gold surface. Corn et al. also applied the system for oligonucleotide arrays [52] and DNA hybridization detection [27, 52]. Later, Nelson et al. [28] improved the performance of intensity SPR imaging by utilizing near infrared (NIR) excitation wavelength. High-contrast SPR images have been shown in the NIR wavelength range (800–1,152 nm). The utilization of incoherent white source and narrow band-pass filter also eliminates the laser fringes that have been observed in conventional SPR imaging set-ups [26, 27, 52]. The NIR SPR imaging sensor was further used for quantitative detection of the hybridization adsorption of RNA and DNA oligonucleotides, and the detection limit was found to be 10 nM [29]. In the same year, the system was integrated with a poly(dimethylsiloxane) (PDMS) microfluidic channels for the fabrication and detection of one dimensional (1D) and two-dimensional (2D) DNA hybridization arrays [53]. The NIR SPR imaging set-up has further been demonstrated for epitope–antibody [54], protein–carbohydrate [55], protein–protein [56], protein–DNA [56], and protein–aptamer [57] interactions detection and protein biomarker screening
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Fig. 1 a Reflection intensity a function of wavelength at fixed incident angle for samples with different refractive index values. “Reprinted with permission from (Jiří Homola et al., Methods, v37(1), pp. 26–36 (2005) [2]). Copyright (2005) Elsevier.” b Intensity SPR image obtained for the specific adsorption ofβ2m (50nM). “Reprinted with permission from (Hye Jin Lee et al., Anal. Chem., v78, pp. 6504–6510 (2006) [105]). Copyright (2006) American Chemical Society.” c Spectral SPR image for an array of different refractive index samples (the scale bar represents 1 mm in length). “Reprinted with permission from (Chi Lok Wong et al.,
Optics Express v19 (20), pp. 18965–18978, (2011) [39]). Copyright (2011) OSA.” d Phase SPR image of MUAM monolayer and bare gold surface. The intensity distributions refer to the interference fringe pattern. “Reprinted with permission from (Aaron R. Halpern et al., Anal. Chem., v83, pp. 2801–2806 (2011) [63]). Copyright (2011) American Chemical Society.” e SPR image with polarization contrast. The vertical bands correspond to referencing areas. “Reprinted with permission from (Marek Piliarik et al., Sensors and Actuators, v134, pp. 353-35(2008) [47]). Copyright (2008) Elsevier”
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[58]. Following this, Wark et al. [58] presented long-range surface plasmon (LRSP) imaging. By using a symmetric dielectric arrangement adjacent to the gold thin film, LRSPs possess longer surface propagation length, higher electric field strength, and sharper angular resonance curve than conventional surface plasmon wave. About 20 % response enhancement has been shown experimentally in DNA hybridization adsorption detection. In recent years, they enhanced the detection limit of SPR imaging by using metallic nanoparticles [59–62]. Gold nanoparticles were adsorbed onto gold diffraction grating at the sensing surface [59, 60], and this detection approach has been found to improve the sensor sensitivity. Detections of DNA at concentration of 10 fM [59] and microRNA [60] have been demonstrated. Silica-coated gold nanorod was further used to enhance the sensitivity of SPR imaging for DNA microarray detection [61]. The sensitivity was found to be 10–100 amol of polymerase product which is equivalent to 0.25 % of a monolayer [62]. Recently, Halpern et al. presented an SPR phase imaging system for ssDNA oligonucleotides detection [63, 64], and further discussion on the work will be provided in the section on phase SPR imaging sensors. Fu et al. [65, 66] reported an SPR imaging system operating at different wavelengths with tilted interference filter [65]. The measurement data were averaged from an area of 400 pixels for 100 images, and the detection limit was found to be 3×10−5 refractive index until (RIU) [66]. Shumaker-Parry and Campbell further demonstrated a high-throughput SPR imaging sensor with 120 sensor sites in a detection limit of 1.8× 10−5 RIU [67]. Recently, Kihm et al. [68] applied the intensity SPR imaging to image the near-field fluidic transport properties within 100 nm from the metal surface. Intensity SPR imaging has been commercialized by GWC Technology (USA), and near infrared imaging technique has been adopted to enhance the sensor response.
Angular SPR Imaging Sensors In angular SPR imaging, the reflection intensity variations for a range of incident angles are scanned. The angular SPR curves and absorption minimum are then calculated for different refractive index samples or molecular binding events. Ruemmele et al. [32] reported an automated angle-resolved SPR imaging sensor which provided a wide dynamic range from 1 to 1.4 RIU based on the variation in excitation angle. Recently, Zhou et al. [69] used a high-precision piezoceramic motor to control and scan the intensity SPR images at different incident angles. The system could distinguish single mismatch in caspase-3 DNA. Angular SPR imaging sensor has also been commercialized by IBIS Technology B. V. (Netherlands) [70] (http://www.ibis-spr.nl/.).
Phase SPR Imaging Sensors Nelson et al. introduced the phase sensitive SPR detection in 1996 [49], which is proved to provide three times higher resolution compared to conventional detection based on angular and wavelength modulations. In 1998, Kabashin and Nikitin [40] demonstrated the first SPR imaging sensor based on phase shift measurement. The probe and reference beam was made to interfere in a Mach–Zehnder interferometer, and the phase information over the sensing surface was captured by a CCD camera. The sensor resolution was found to be 4×10−8 RIU for gas detection. Nikitin et al. [41] later used a birefringent plate to split the p- and s-polarization beams laterally, and the overlapping parts of these two beams were allowed to interfere after passing through an oblique polarizer. These two beams shared almost the same optical path, and the vibration noise during phase detection was suppressed. Ho and Lam [71] performed fringe shift analysis between the signal and reference SPR interference patterns captured from a dual channel chamber, and the sensor resolution was found to be 10−5 RIU. Su et al. [72] then demonstrated a common-path phase shift interferometry-based SPR imaging sensor. A liquid crystal phase retarder was used for phase modulation, and the phase information was unwrapped with a five-step phase shift reconstruction algorithm [72, 73]. The sensor resolution was found to be 2×10−7 RIU in nitrogen and argon gas detection. After that, Yu et al. reported two designs of phase SPR imaging sensor [74–76]. In the first approach, the p- and spolarized beam was separated and they were allowed to interfere after passing through a polarizing prism [74, 75]. The fringe displacements give the phase information, and a detection limit of 3×10−5 RIU was demonstrated [75]. The second approach utilized an electro-optical crystal to perform time domain phase modulation [76], and the Stoilov algorithm [77] was used for phase extraction; however, the sensor resolution was limited [76]. Recently, Halpern et al. [63] used the polarizer-quartz wedge depolarizer combination to create interference fringe image of the sensing surface, and the interference fringe shifted according to the adsorption of biomolecules. It has been further combined with nanoparticle-based detection for signal enhanced [64], and the detection limit in short single stranded DNA oligonucleotide measurement was found to be 25 fM. In the past decade, Wong and Ho et al. have reported a series of phase SPR imaging sensors [42–45, 78–81]. In 2005, they invented a multi-pass SPR phase imaging sensor [44]. The SPR sensor head was located in an optical cavity, and the excitation beam was allowed to be incident on the sensor surface for multiple times, which enhanced the resultant phase shift in the interference image. A phase shift improvement of a factor of 3 was demonstrated, and a US patent has been filed
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for this technique [44]. It has further been applied for protein– aptamer detection [80, 81]. Conventional phase SPR imaging sensors mainly relied on interference fringe analysis, and a large portion of spatial information has been sacrificed in the phase extraction process [44, 63, 64, 71, 74–76]. Wong et al. presented a 2D SPR phase imaging sensor technique [43]. A piezoelectric transducer was used to modulate the phase of the interference image in the time domain, which allowed every pixel on the SPR image to be converted into corresponding phase shift values, and a 2D SPR phase map could be obtained without the use of expensive electro-optical modulators. In 2008, they applied a differential phase measurement scheme together with the 2D phase mapping technique for bio-molecular array detection [45]. At surface plasmon resonance, the phase of the p-polarized light is altered, while the phase of the s-polarized light remains unchanged [3, 45]. A phase difference is therefore created between the p- and s-polarized light. In this work, they separated the light interference pattern into the p- and s-polarized images and the calculation of differential phase eliminated the common optical path noise and thus enhancing the sensor resolution. A detection limit of 8.8×10−7 RIU has been demonstrated [45].
Polarization Contrast-Based SPR Imaging Sensors Piliarik et al. [46] reported a SPR imaging sensor based on polarization contrast. The SPR sensing head was placed between two polarizers with perpendicular orientation. At surface plasmon excitation, a phase shift was introduced between the p- and s-polarized light and the intensity of the SPR image was increased. A detection limit of 3×10−6 RIU was reported. Later, they improved the sensor resolution by one order of magnitude (2×10−7 RIU) [47] with the subtraction of the dark current signal and intensity fluctuations of the light source. The system has further been applied for the detection of oligonucleotides [82], protein biomarker in diluted blood plasma [83], and nucleic acids identification [84]. Patskovsky et al. [85] described a scheme of spatially modulated surface plasmon resonance (SPR) polarimetry. A birefringent wedge was utilized to produce periodic changes of phase relations between the p- and s-polarized light, and they were allowed to interfere after passing through an analyzer at 45° orientation. The Fourier transform method was used for phase extraction. Recently, Han et al. [86] presented an ellipsometric SPR imaging system, and the prism-based SPR sensor head was located between a polarizer and an analyzer. The analyzer was rotated at three different angles, and the subsequent intensity images were used to calculate the phase and ellipsometric parameters. The sensor resolution was found to be 1.25× 10−6 RIU.
Spectral SPR Imaging Sensors Wong et al. first demonstrated real-time 2D spectral SPR imaging in 2003 [87]. The p-polarized polychromatic light source was used as the excitation source. The SPR absorption occurred at particular resonance wavelength, and the resonance wavelength shifted for different refractive index mediums; therefore, corresponding spectral profiles were produced at the spectral SPR image. This imaging approach has been applied for 2D refractive index mapping in elastohydrodynamic lubricate (EHL) contacts [35–38]. They also demonstrated the first pixel to pixel color quantification in spectral SPR image with the Hue extraction algorithm [35–38, 93–95]. It provided full resolution 2D information, which was essential for high-throughput array detection. This imaging technique was compared with the conventional optical interferometry method used in EHL studies [37], and two orders of magnitude (180 times) improvement in accuracy has been shown. In 2006, Yuk et al. also reported an SPR imaging sensor with wavelength interrogation [88]. The optical fiber probe of a spectrometer was required to scan over the sensing surface, and the SPR absorption minimum values at every scan point were integrated to form an SPR image. The scanning time for a 2-mm spot was 180 s, and the sensor resolution was limited (7.6×10−5 RIU). It has been applied for protein array [88] and C-reactive protein binding detection [89]. Liu et al. [90] later presented a parallel scan spectral SPR imaging technique. A cylindrical lens was used to focus a line-shaped light illumination on the sensing surface, and the SPR line image was diffracted by a diffraction grating and then projected on a CCD camera. It was a 1D scanning technique, and the resolution was limited to 8.1×10−5 RIU. Bardin et al. [91] demonstrated a similar optical system with the application of the double polynomial fit technique [91], and the sensor resolution was further improved to 3.5×10−7 RIU. However, the imaging system only provided 1D resolution. Recently, Lee et al. [92] presented a nanohole array-based SPR imaging sensor. White light source was used to excite the surface plasmon resonance at the substrate, and the 1D LSPR line image was processed with an imaging spectrometer and captured with a low-noise CCD camera. The detection limit was found to be 7.7×10−6 RIU. To date, majority of existing spectral SPR imaging sensors can only provide 1D spatial resolution [90–92] and timeconsuming scanning is required [88–92], while real-time imaging and 2D resolution are two important requirements in high throughput micro-array detection. In addition, the sensor resolutions are limited in existing spectral imaging systems [88–90, 92]. In this context, we combine the spectral and phase interrogations and present a new type of spectralphase SPR imaging sensor (spectral-phase SPRi) [34, 39]. It
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provides unique real-time 2D colorimetric SPR imaging for high throughput micro-array detection and operates with the sensitive phase response, while complicated and timeconsuming phase modulation is avoided. The spectral-phase SPR imaging sensor measures the spectral characteristic variation caused by the steep phase change occurring at surface plasmon excitation. As shown in Fig. 2b, an SPR prism coupler is placed in between two polarizers with perpendicular transmission axes and the transmission of an incident beam is forbidden. At the excitation wavelength of surface plasmon, a phase difference is introduced between the p- and s-polarization component of the light. It rotates the orientation angle of the polarization ellipse (Fig. 2a), and the light interacting with the surface plasmon is allowed to pass through the crossed polarizers. As the momentum of the surface plasmon wave only matches with a particular wavelength range, a particular spectral profile is produced, which is associated with the steep phase response at the surface plasmon excitation. This method enhances the sensitivity of conventional spectralbased SPR sensor through probing the steep phase response at the surface plasmon resonance. The sensor resolution has been characterized in a refractive index sensing experiment with different concentrations of salt solutions ranging from 0 to 7 %, which corresponds to refractive index values in 1.3330–1.3454 RIU. The spectral-phase SPR images are shown in Fig. 3a–f. The SPR image for water sample (0 %) is red in color, and the green component in the SPR image increases with increasing refractive index values. The corresponding spectra are shown in Fig. 3g, which indicates clear spectral profile variations for different concentrations of salt solutions. During image processing, the hue component in the HSV color space [93–95] is used to quantify the color variations in the spectral SPR images Fig. 2 a Ellipse of the elliptically polarized light E′. b Schematic diagram of the spectral SPR imaging sensor based on polarization orientation. “Reprinted with permission from (Chi Lok Wong et al., Biosensors and Bioelectronics v47, pp. 545– 552, (2013) [34]). Copyright (2013) Elsevier”
[36–38]. It enables pixel-to-pixel information conversion, and full resolution 2D spectral SPR image can be obtained in real-time, which is not available with existing scanningbased spectral SPR imaging sensors [88–92]. The hue responses of the SPR images (Fig. 3) are extracted and plotted against the refractive index values in Fig. 4. This gives the response curve of the spectral-phase SPR imaging sensor. In addition, the measurement standard deviations (SD) between five averaged data are given in Table 1. To consider the overall measurement SD value, at 0.032 hue unit, as the measurement stability of the sensor, the sensor resolution was found to be 1.6×10−6 RIU [39]. The sensor resolutions of existing spectral SPR imaging sensors as reported in [88, 89], [90], and [92] are 7.6×10−5, 8.1×10−5, and 7.7×10−6 RIU respectively. It clear shows about one order of magnitude improvement in sensor resolution. resolution ¼
RIU range measurement S:D: responseðΔhueÞ
ð4Þ
Figure 5a further shows the spectral-phase SPR image for an array of refractive index samples (1.3333, 1.3365, and 1.3454 RIU), and the color texture variation has further been quantified to a 2D hue map. Figure 5b, c is the spectral SPR image shown in [91] and [89], respectively. Majority of existing spectral SPR imaging sensors [90–92] rely on the combination of diffraction grating and CCD camera for capturing the whole spectrum. This approach limits the SPR image to a 1D line format (Fig. 5b) profile. Figure 5c is the SPR image integrated by the SPR absorption minimum wavelength values. In each pixel of the image, the fiber probe of a spectrometer was used to record the SPR spectrum and the absorption dip wavelength was determined [88, 89]. The scanning time for a 2-mm spot was 180 s, and the spatial resolution is limited by the physical
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Fig. 4 The response curve of the spectral SPR imaging sensor in the refractive index range between 1.3333 and 1.3454 RIU. The error bar is obtained from the SD between five averaged measurement data. “Reprinted with permission from (Chi Lok Wong et al., Optics Express v19 (20), pp. 18965–18978, (2011) [39]). Copyright (2011) OSA”
Biosensor Array To demonstrate array-based biosensing with the spectralphase SPR imaging technique, the specific binding between bovine serum albumin (BSA) antigen and antibody has been measured. Protein array as described in Fig. 6a was fabricated on the sensing surface. Glucose oxidase and blank sensor spots served as negative controls in the experiment. The protein array was first kept in PBS buffer for base-line detection. After that, specific BSA antibody (2.5 μl/ml) was injected to the sensor surface, and the binding interactions were allowed to take place for 1 h. Finally, the protein array surface was washed with PBS buffer for the removal of nonspecific bindings. Figure 6 shows the spectral SPR images taken at different stages of the binding process. Comparing the SPR image taken with PBS buffer (Fig. 6b) and after the injection of specific antibodies (Fig. 6f), the color of the specific sites (A1, A4, B2, B3, C2, C3, D1, and D4) has varied
Fig. 3 Spectral SPR images for different concentrations of salt solutions. a Water (0 %). b 1 % salt solution. c 2 % slat solution. d 3 % salt solution. e 4 % salt solution. f 7 % salt solution. g Spectrums measured for different concentrations of salt solutions ranged from 0 to 7 % (normalized with s-polarization). “Reprinted with permission from (Chi Lok Wong et al., Optics Express v19 (20), pp. 18965–18978, (2011) [39]). Copyright (2011) OSA”
size of the fiber head. Nevertheless, our spectral SPR image shown in Fig. 5a is a full-resolution 2D image, in which no scanning is required and the response is in real time. It is an ideal technique for real-time high-throughput protein/DNA microarray imaging detection.
Table 1 The measurement standard deviation of different concentration salt solutions 0% 1% (water)
2%
3%
4%
7%
Refractive index 1.3333 1.3347 1.3365 1.3383 1.3400 1.3454 (RIU) Measurement 0.030 0.023 0.073 0.023 0.024 0.017 standard deviation (hue unit 0–255) “Reprinted with permission from (Chi Lok Wong et al., Optics Express v19 (20), pp. 18965–18978, (2011) [39]). Copyright (2011) OSA”
Plasmonics Fig. 5 a 2D spectral SPR image of an array of refractive index samples (water (0 %), salt solution (2 %), and salt solution (7 %)). Applying the hue extraction, the color distribution is quantified. The responses of 0, 2, and 7 % salt solution spots are 25.7, 69.1, and 83.4 (hue unit) (average value from all sensing sites). (The scale bar represents 1 mm in length). “Reprinted with permission from (Chi Lok Wong et al., Optics Express v19 (20), pp. 18965–18978, (2011) [39]). Copyright (2011) OSA.” b 1D spectral SPR image of a biochip reported in [91]. “Reprinted with permission from (Fabrice Bardin et al., Biosensors and Bioelectronics, v24(7), pp. 2100– 2105 (2009). [91]). Copyright (2009) Elsevier.” c SPR image integrated by the SPR absorption minimum wavelength values. In each pixel of the image, the fiber hand of a spectrometer was used to record the SPR spectrum and the absorption dip wavelength was determined [88, 89]. The scanning time for a 2-mm spot was 180 s, and the spatial resolution is limited by the physical size of the fiber hand. The first SPR image was Au, and the final SPR image was anti-CRP on the Au/DTSP/CRP surface. “Reprinted with permission from (Jong Seol Yuk et al., Sensors and Actuators B: Chemical, v119, pp. 673–675 (2006) [89]). Copyright (2006) Elsevier”
a
b
c
from red to green due to the specific binding between BSA antigens and antibodies. However, no significant color variation (molecular binding) was seen in the negative control sites (A2, A3, B1, B4, C1, C4, D2, D3) and blank sites (E1–E4). The color texture variations in the SPR images were further quantified with the hue component as shown in Fig. 7.
SPR absorption minimum wavelength (nm)
Fig. 7a, b are the SPR image taken with PBS buffer, and Fig. 7c, d are the image captured at 1 and 5 min after the injection of the specific BSA antibody (2.5 μl/ml), respectively. Clear responses have been shown in the specific sites of the spectral SPR image within 5 min of reaction (Fig. 7d). Figure 7e–h shows that the signals in all specific sites
Plasmonics Fig. 6 a The protein array consists of three different array elements: BSA (red spots, positive sample), glucose oxidase (GOx) (black spots, negative sample), and blank sites (white spots, background control). Spectral SPR images for specific BSA antigen-antibody binding detection (the scale bar represents 1 mm in length). b At the beginning of the experiment, the protein array was kept in PBS buffer. c (46 min) 15 min after the injection of anti-BSA. d (90 min) 1 h after the injection of BSA antibody. e (106 min) 15 min after the PBS buffer washing process. “Reprinted with permission from (Chi Lok Wong et al., Biosensors and Bioelectronics v47, pp. 545–552, (2013) [34]). Copyright (2013) Elsevier”
increased against time, when increased amounts of BSA antibodies bound onto the protein array. The quantification of the SPR images also provides a series of binding curves for all sensor sites in the protein array, and they are shown in Fig. 8. The sensor resolution for biomolecules can be calculated from the following [4]. Detection limit ¼
Concentration of bio‐molecule Measurement stability Sensor response
ð5Þ It is because that the injection of 2.5 μl/ml BSA antibody produces an averaged overall response of 39.13 (hue unit) and
the overall measurement S.D. is 0.13 (hue unit), the biosensing resolution is found to be 8.26 ng/ml (125 pM). This value is 12.1 times and 93.2 times better than the sensor resolution reported in [96] and [45], respectively, for IgG and BSA antibody-antigen binding detections with phase SPR imaging.
Diffraction Grating-Based SPR Imaging Sensors Diffraction grating coupler is not as widely used as the prism coupler in SPR imaging for the excitation of surface plasmon [2–5]. The working principle of grating-based SPR sensing is described in Fig. 9a. Excitation light is
Plasmonics Fig. 7 2D hue profiles extracted from the spectral SPR images. a At the beginning of the experiment, the array was kept in PBS buffer. b (30 min) the array was kept in PBS buffer. c (32 min) 1 min after the injection of anti-BSA (2.5 μl/ml in PBS). d (36 min) 5 min after the injection of anti-BSA. The average response in the specific BSA antigen sites is increased from 0.19 (hue unit) to 13.95 (hue unit), which is equivalent to 66.21 % increase. However, less than 3.05 % increases are recorded in the non-specific GOx sites. e (46 min) 15 min after the injection of anti-BSA. f (61 min) 30 min after the injection of antiBSA. g (76 min) 44 min after the injection of anti-BSA. h (90 min) 59 min after the injection of antiBSA. i (105 min) 15 min after array surface washing with PBS buffer. Less than 1 % signal decrease is recorded in the specific sites (i). It reveals the specificity of the binding interactions. Comparing the 2D Hue profiles shown in i and a, 162.1 % increase is found in the specific BSA antigen sites, while only 4.45 % increase is indicated in the nonspecific GOx sites (the scale bar represents 1 mm in length). “Reprinted with permission from (Chi Lok Wong et al., Biosensors and Bioelectronics v47, pp. 545–552, (2013) [34]). Copyright (2013) Elsevier”
Plasmonics Fig. 8 Binding curves for all sensor sites in the array. a, d, f, g, j, k, m, and p reveal the binding curves of the specific BSA antigen sites, and the binding curves of the non-specific GOx sites are illustrated in b, c, e, h, i, l, n, and o. Specific bindings are clearly shown in the specific BSA antigen sites (A1, A4, B2, B3, C2, C3, D1, D4), while less than 5 % variations are recorded in the nonspecific GOx sites (A2, A3, B1, B4, C1, C4, D2, D3). At the equilibrium stage, the average response recorded in the specific BSA antigen sites is 24.8 times higher than that of the nonspecific GOx sites. In addition, it is 6.2 times higher than that of the blank control sites (q–t, E1–E4). “Reprinted with permission from (Chi Lok Wong et al., Biosensors and Bioelectronics v47, pp. 545–552, (2013) [34]). Copyright (2013) Elsevier”
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incident onto a gold/silver coated diffraction grating, and the surface plasmon propagating at the metal-dielectric interact will be excited when the momentum of the diffraction light matches with that of the surface plasmon wave, which is described in Eq. (6) [4], 2π 2π nsinθ þ m ¼ k sp λ Λ
ð6Þ
where Λ is the period of the diffraction grating, m is an integer, and ksp is the wave vector of the surface plasmon wave. Diffraction grating-based SPR sensing was first demonstrated in 1987 by Cullen et al. [97]. A gold-coated grating was used for the excitation of surface plasmon at the metal– dielectric interface. The reflection intensity was plotted as a function of incident angle, and the shift of the angular SPR dip corresponded to the bio-molecular binding. Grating coupler was then demonstrated for imaging detection in 2001 by Brockman and Fernandez [98]. In their design, monochromatic light at 860 nm was illuminated on a gold-coated diffraction grating and 400 sensing channels were allowed to operate in
Fig. 9 a Working principle of diffraction grating based angular SPR imaging sensor. b Imaging of an array of SPR angular spectra from a row of diffraction gratings [100]. “Reprinted with permission from (Jakub Dostálek et al., Sensors and Actuators B v107, pp. 154–161, (2005) [100]). Copyright (2005) Elsevier.”
parallel. This approach was commercialized by HTS Biosystems in 2005 [99], and the technique was further acquired by Biacore International AB in the same year. Homola et al. reported another design of highthroughput SPR sensor with an array of minimized diffraction grating spots (216 elements) [100]. A 635-nm laser diode was used as the monochromatic excitation source. It was focused on each row of diffraction grating with scanning optics, and the reflected angular spectrums were captured with a CCD detector. The detection limit was found to be 5 × 10 −6 RIU. The system was further applied for multiplexed protein-analyte scanning [101], and the detection limit was improved to 5×10−7 RIU with reduced sensing sites (120). They then developed a portable device with a gold-coated grating sensor chip [102]. In the optical configuration, ten angular spectrums captured from ten sensing channels were projected on the CCD detector with a cylindrical lens and the resonance angle shifts indicated the refractive index changes or biomolecular bindings. Microfluidic sample delivery system,
Plasmonics
heat insulation, and cooling system are integrated in the device. The refractive index resolution was found to be 6×10−7 RIU. Detection of the hybridization of oligonucleotide probes has been demonstrated, and the detection limit was found to be 1 nM. Singh et al. reported the use of gold-coated commercial compact disk (CD) as grating substrate for SPR imaging measurement [103]. Array of functionalized monolayer layers were spotted on different regions of the CD-grating and measurements on protein bindings (bovine serum albumin) have been performed with the system. Unfricht et al. also developed a grating-based imaging system, which relies on angle scanning of SPR dip [104]. Recently, the system was applied for CD4 + T cells detection which found applications in disease diagnostics [106].
Conclusion Since the first invention of surface plasmon microscopy by Rothenhäuslar and Knoll in 1988 [1], SPR imaging has found rapid increasing research interests [2–5] and wide applications in drug discovery [6], biomarker screening [7], nucleic acid detection [5], food safety [8, 9], and environmental monitoring [10] in the past two decades. SPR imaging sensors based on intensity [1, 26–31], angular [32, 33], wavelength [34–39], phase [40–45], and polarization [46–48] modulations have been reported to date. However, intensity-based SPR imaging sensors are the most widely used method [4, 26–31]. Among different detection approaches, phase SPR imaging [40–45, 49, 63, 64, 71–81] are so far the most sensitive technique; however, the practical usage has been limited by the sensitivity to background noise. In this context, we combine the spectral and, phase interrogation and demonstrate the multiplex spectral-phase SPR imaging biosensor. It enhances the sensitivity of conventional spectral based SPR sensor through probing the steep differential phase response at the surface plasmon resonance between the p- and s-polarization. One order of magnitude improvement in sensor resolution has been demonstrated with this imaging sensor compared to existing spectral SPR imaging sensors [88–89, 92] and phase SPR imaging sensors [45, 96]. Our spectral SPR imaging sensor also provides real-time 2D resolution imaging, while only 1D resolution is enabled with the reported spectral SPR imaging techniques [90–92]. Such imaging sensors can find promising applications in clinical disease diagnosis, protein biomarker, and drug screening. Acknowledgments This project is supported by Singapore BioImaging Consortium. Acknowledgment also goes to Li-Tin Ho for the contribution in experimental design and ideas.
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