One unique feature of biosensors and those related to SPR in particular is the multidisciplinary approach needed to create novel designs, especially with regards to instrumentation. With the help of engineers and physicists the advancement of biosensors in terms on sensitivity is being attacked not only from a surface and chip design point of view, but also that of advanced optics, light sources, and detectors.
Typically surface plasmons are excited in the Kretschmann configuration by either a light-emitting diode or laser, but recently advances in light source technology have created a new class of instrumentation based on a variety excitation sources. One recent group used a white light source for a dual-functioning SPR biosensor composed of surface plasmon microscopy (SPM) and SPR spectroscopy (Yuk et al. 2008
). Using this setup SPR images and spectra were obtained from protein arrays under the same measurement environment by the dual-function SPR biosensor installed with a CCD camera and a spectrometer on a titled angle plate. As proof-of-principle for the design, interactions of glutathione surface with glutathione S-transferase fusion proteins on arrays were analyzed by SPR image contrasts as well as by SPR spectroscopy. Also using a white light source the development of a wavelength-resolved SPR system was demonstrated recently (Chen et al. 2008
) showing a very high sensitivity to low concentrations of uranyl ion from the micromolar to the picomolar range. The binding of uranyl ion to an interfacial recognition layer induces the SPR wavelength shift, permitting a sensitive detection of uranyl ion. Consequently, these results indicate that using this wavelength-resolved SPR spectroscopy system is very useful for the study of small molecular interactions.
Rather than using a different light source, the use of a radially polarized beam to excited surface plasmons has been reported (Chen and Zhan 2009
). With this excitation method surface plasmons can be excited in all directions propagating to the geometric center, constructively interfering with each other and generate a strongly focused evanescent non-diffracting Bessel beam. In this work the authors demonstrate the excitation of surface plasmons on a silver-glass interface with images captured by a CCD camera. A dark ring corresponding to surface plasmon resonance excitation by a focused radially polarized beam is observed. While not applied to a biosensing platform yet, this instrumental design with the radially polarized beam offers a glimpse of a possible new light source for SPR biosensing.
Another light source that is commonly employed for SPR and SPR imaging application is a light emitting diode or LED as demonstrated in our lab and others (Suzuki et al. 2005
; Wilkop et al. 2004
; Yanase et al. 2010
; Yang and Cho 2008
). Specifically, the creation of an SPR imager using a radially polarized illumination from a LED at 530 nm to obtain speckle-free images with high spatial resolution along all orientations was recently reported (Vander and Lipson 2009
). The sensitivity to refractive index changes for a saline solution was estimated to be better than 10−3
RIU. Another type of LED that has become more prominently used is an organic light emitting diode or OLED. One group has used an integrated OLED to create a novel SPR sensor (Frischeisen et al. 2008
; Frischeisen et al. 2009
). In this instrument the light emitted by the OLED leaves the prism after reflection at the sensing layer and passes a linear polarizing filter before it is focused by a collimating lens onto an optical fiber guiding it to a spectrometer. The basic functionality of the sensor was demonstrated by measuring the spectral and angle-dependent surface-plasmon dispersion at metal/air interfaces for sensing layers consisting of silver and gold with different thicknesses. As measured by refractive index changes with NaCl solutions, the instrument exhibits sensitivity on par with commercial instruments.
Not only can new light sources be used for enhanced sensitivity or resolution, but they can also be used for speed of data acquisition. Recently the development of a novel technique that has the potential to realize interrogation of SPR sensors at very high speed was reported (Zheng et al. 2008
). A broadband coherent laser generating short optical pulses at a high repetition rate is used along with a highly dispersive optical element to obtain fast data acquisition. In their setup the change in pulse shape over time is measured with a photodetector. The SPR response could be acquired for each pulse with data acquisition conceivably possible at a rate of tens of MHz, vastly improving current SPR techniques. By measuring the variations in the pulse shapes of the chirped pulses, sensitive SPR measurements in PBS buffer were made to demonstrate the utility of this apparatus.
Additionally, beyond acquisition speed and sensor sensitivity, recently a group utilized an untapped area of the electromagnetic spectrum by the advent of the first SPR sensor in the mid-infrared range (Herminjard et al. 2009
). Surface plasmons are excited on a Ti/Au substrate deposited on a CaF2 prism where light excitation is provided by a quantum cascade laser (QCL) source. Evidence of SPR is presented by detecting CO2
mixtures as test samples. Due to the absorption of CO2
at this wavelength in the mid-IR, it is shown that the sensitivity of this configuration is five times higher than a similar SPR sensor operating in the visible range of the spectrum. This instrument represents a promising detection tool for numerous gaseous molecules that absorb in the IR region.
Another area of great interest, especially for engineers, is the development of new SPR or SPR imaging instruments. Due to the low flexibility and limited possibilities offered by commercially available SPR imagers for sensor development, many groups (including ours) choose to build their own instruments. Among the most interesting developments in the last few years includes a two-dimensional phase-detection system for a SPR imaging (Lee et al. 2008
). The sensor utilizes polarization interferometry to detect phase differences between the s and p polarizations. This instrument can successfully detect a spatial phase-difference variation in a 1 mm2
area with a sensitivity of 4.3×10−6
RIU using PBS buffer as a calibration standard. Another similar instrument has been reported using polarization interferometry (Yu et al. 2008a
; Yu et al. 2008b
). The authors noted the complete common-light path detection method can restrain light noise and improve detection sensitivity. For sensitivity determination various concentrations of NaCl solutions were used indicating a sensitivity of 10−6
refractive index units (RIU).
There is also evidence of slightly different experimental protocols for obtaining SPR imaging data can lead to enhanced sensitivity. Specifically, the detection limit of SPR measurements could be improved by a factor of 2–3.5 if the angle of incidence is near the reflection minimum of the SPR resonance curve instead at the position of the steepest slope, the standard alignment in SPR imaging (Zybin et al. 2007
). The enhancement of the detection power, a result of signal-to-noise (S/N) optimization, is demonstrated by applying a photodiode and a CCD camera for SPR detection while using NaCl solutions as calibrants. More instrumental optimization has been reported by Mirsky and co-workers who report enhanced analytical performance of SPR imaging by splitting a macroscopic sensing surface into multiple microscopic neighboring sensing and referencing subareas (Boecker et al. 2008
). Their data indicates multiple referencing reduces intensity fluctuations across the total sensing area and, therefore, improves the S/N ratio proportional to the splitting factor. This data acquisition method is demonstrated by examining the biotin/streptavidin interaction resulting in enhanced sensitivity. Specifically, an effective variation of the reflected intensity of about 10−4
, which corresponds to the refraction index variation of 3×10−6
, was detected with S/N ratio about 10 without any temperature stabilization of the sensing area. These are just a few of instrumental enhancements that could lead to remarkable improvement in terms of SPR imaging and spectroscopy sensitivity in the future.