Preconcentrator Integrated a Periodic Metallic Nanoslit-based Surface Plasmon Resonance (SPR) Sensor Using Nanoimprinting Lithography for Immunoassay

In this study, a high sensitivity biosensing platform by integrating nanofluidic preconcentrator with periodic metallic surface plasmon resonance sensor has been developed. The target protein, was concentrated to enhance sensitivity of immunoassay. Firstly, the concentrated protein plug was trap in the sensing area of slit-based Surface Plasmon Resonance (SPR) in a microfluidic channel by electrical potential difference. The antibody-antigen interaction on periodic metallic slits resulted in, a red-shift of the resonant spectrum signals which is corresponding to the numbers of the antibody-antigen conjugation. The periodic nano-grating structure were clarified and fabricated on a silicon wafer as a mold by E-beam lithography and reactive ion etching. The nanostructure was transferred onto a cyclic olefin polymer (COP) by nanoimprint lithography. The gold was deposited on the grating structure of COP by sputter. A porous material, Nafion, was used as the ion-selective channel and, was aligned to the metallic grating structure on COP. Microchannels were made by polydimethylsiloxane (PDMS) using soft lithography process. After the chemically modified surface treatment, the COP can be bond with PDMS by oxygen plasma. The metallic SPR sensor was located in one of parallel microfluidic channels, which were located cross over the Nafion channel. In this study, bovine serum albumin (BSA) and anti-BSA were used as the testing samples. The disulfide bonds on it would form covalent bonds with Au and is able to be detected by spectrometer. Thereafter, 20 ng/mL of BSA antibodies was introduced into the concentration channel. Then, by adjusting electrical potential, the antibodies were condensed on the surface plasmon resonance chip. The concentrated magnification can be sufficiently derived by the red-shifted value deference and the reference curves of BSA antibody concentration. In summary, the concentration fold can be raised up to approximately 10,000 folds as the 20 ng/mL of antibodies was condensed to 200 µg/mL. Thus, we can conclude that the minimal detectable limit is about 2 pg/mL. In conclusion, the nanoimprint lithography has several advantages, such as, mass production, low cost, and time saving. Furthermore, the preconcentrator has significantly broken through regarding the detectable limit and SPR has the advantages of high sensitivity, real-time detection, and label-free features. By integrating these two techniques, we can have a label-free, micro-concentration detectable platform by using simple measurement system.