The proposed apparatus is a tunable surface-plasmon resonance sensor capable of switching between imaging and angular resolved spectroscopy without moving parts, all with high accuracy and aberration corrections. It includes a source of coherent light, a collimating/focusing tunable lens, adaptive optics, a glass prism optically coupled with a glass/metal chip, a (tunable) objective lens and a two dimensional detector. The employ of electrically tunable lenses (ETL) permits movements free and fast changing of the focal point of the system between infinite and some centimeters, switching in this way between two complementary but different configurations. Moreover, the optional insertion of adaptive optics components allows a finer tuning of the focal point and the compensation of the aberrations, which may be introduced by the tunable lens.
Besides switching easily between different configurations, this new setup allows also changing the illumination pattern on the sensing surface. This permits to gain more information with the same setup and during the same measurement session. For example, a plane wavefront allows having an image of the entire sensor surface and making time resolved measurements. As one is instead interested in having a local information of the behavior of the SPR response, the beam can be focused on the back of the prism in order to have a quick view of the response as function of the incident angle.
Our SPR sensor setup is in the Kretschmann configuration. It employs a monochromatic light source properly filtered and TM-polarized, a prism to couple the radiation with the surface plasmon at the metal/dielectric interface (the material of the prism depend upon the metal used, the working wavelength, and the final application) and a bi-dimensional detector (CCD or CMOS).
The core and novelty element of our apparatus is that a tunable optical element is inserted in the optical layout. Our set up allows to:
• perform complementary measurements (SPR angular resolve and SPR imaging) on the same setup with no moving parts;
• calibrate the imaging thanks to the angular resolved measurement;
• enhance the quality of the image and SPR curve;
• change/optimize the penetration depth of the evanescent wave probe at the interface metal/target.
Amongst the sensors based on bio-affinity mechanisms, those that use surface plasmon resonance as a transducer system are of great importance The undoubted advantages of such a technique are the very high speed of analysis (chemical manipulations are not required), the high reproducibility and the high sensitivity relative to other techniques.
Generally speaking, an SPR optical sensor comprises an optical system, a transducer that relates the optical phenomena to those that are biochemical, and an electronic system that carries the optoelectronic components of the sensor and allows the processing of the data. The transducer transforms variations in concentration of a solution into variations of refractive index, which can be determined optically by investigating the position of the resonance peak. Such a system is an “aspecific” sensor owing to the fact that it measures variations in the refractive index, and it can become specific on the basis of the application requirements.
Another technique for SPR sensing consists of SPR imaging (SPRi), which has been developed for simultaneously analyzing up to around a hundred molecular interactions on a device referred to as a “plasmonic biochip”. In this case, the collimated laser beam is expanded in order to illuminate an wide-area biochip (or on an array of chips) and the signal is detected by a CCD detector.
The apparatus for SPR sensing, invented and developed in our labs, is versatile and of simple construction.
The possible drawbacks relates to the limited lateral resolution of SPR imaging technique, given by the finite plasmon propagation length. However this in an intrinsic characteristic of SPR imaging systems.
- environment
- bio- and medical
- agro-food
Silvia Cella
