Near infrared surface plasmon resonance phase imaging and nanoparticle-enhanced surface plasmon resonance phase imaging for ultrasensitive protein and DNA biosensing with oligonucleotide and aptamer microarrays.

The techniques of surface plasmon resonance-phase imaging (SPR-PI) and nanoparticle-enhanced SPR-PI have been implemented for the multiplexed bioaffinity detection of proteins and nucleic acids. The SPR-PI experiments utilized a near-infrared 860 nm light emitting diode (LED) light source and a wedge depolarizer to create a phase grating on a four-element single-stranded DNA (ssDNA) microarray; bioaffinity adsorption onto the various microarray elements was detected via multiplexed real time phase shift measurements. In a first set of demonstration experiments, an ssDNA aptamer microarray was used to directly detect thrombin at concentrations down to 100 pM with SPR-PI. Two different ssDNA aptamers were used in these experiments with two different Langmuir adsorption coefficients, K(A1) = 4.4 × 10(8) M(-1) and K(A2) = 1.2 × 10(8) M(-1). At concentrations below 1 nM, the equilibrium phase shifts observed upon thrombin adsorption vary linearly with concentration with a slope that is proportional to the appropriate Langmuir adsorption coefficient. The observed detection limit of 100 pM is approximately 20 times more sensitive than that observed previously with SPRI. In a second set of experiments, two short ssDNA oligonucleotides (38mers) were simultaneously detected at concentrations down to 25 fM using a three-sequence hybridization format that employed 120 nm DNA-modified silica nanoparticles to enhance the SPR-PI signal. In this first demonstration of nanoparticle-enhanced SPR-PI, the adsorbed silica nanoparticles provided a greatly enhanced phase shift upon bioaffinity adsorption due to a large increase in the real component of the interfacial refractive index from the adsorbed nanoparticle. As in the case of SPR-PI, the detection limit of 25 fM for nanoparticle-enhanced SPR-PI is approximately 20 times more sensitive than that observed previously with nanoparticle-enhanced SPRI.