Particle sensing with confined optical field enhanced fluorescence emission (Cofefe)

Abstract

We describe the development and performance of a new type of optical sensor suitable for registering the binding/dissociation of nanoscopic particles near a gold sensing surface. The method shares similarities with surface plasmon resonance microscopy but uses a completely different optical signature for reading out binding events. This new optical read-out mechanism, which we call confined optical field enhanced fluorescence emission (Cofefe), uses pulsed surface plasmon polariton fields at the gold/liquid interface that give rise to confined optical fields upon binding of the target particle to the gold surface. The confined near-fields are sufficient to induce two-photon absorption in the gold sensor surface near the binding site. Subsequent radiative recombination of the electron-hole pairs in the gold produces fluorescence emission, which can be captured by a camera in the far-field. Bound nanoparticles show up as bright confined spots against a dark background on the camera. We show that the Cofefe sensor is capable of detecting gold and silicon nanoparticles, as well as polymer nanospheres and sub-μm lipid droplets in a label-free manner with average illumination powers of less than 10 μW/μm.

Sensing Biomolecular Interactions by the Luminescence of a Planar Gold Film

“…In this section, we examine the proposed mechanism of the Cofefe sensor, field confinement upon formation of a nanocavity between the target and the planar film, followed by two-photon excited electron–hole pair generation in the gold film. This mechanism has been suggested because strong luminescent signals were observed when nonfluorescent particles, such as silicon nanoparticles, polystyrene nanoparticles, and lipid droplets, approached the Au surface . Because it is unlikely that nonfluorescent particles produce strong luminescent signals, the Cofefe response has been attributed to excitations in the gold layer itself.…”

“…For particles sized below the diffraction limit of light, the detected spot on the camera reveals a clean radially symmetric pointspread function . This suggests that localization analysis techniques that are used in fluorescence imaging could be implemented to improve the resolution of the Cofefe image. , Enhancing the resolution is useful for increasing the number of particles that can be detected within the field of view, thus enhancing the multiplexing capability of the sensor.…”

“…The green boxed area, shown enlarged in b, highlights an individual particle. The apparent size of the point source is defined by the PSF, with a width that measures ∼0.4 μm . To improve the resolution, we use the SRRF algorithm, based on 100 raw frames acquired at 68 fps.…”

“…A schematic of the optical setup is shown in Figure . A detailed description of the system has been given previously . Briefly, the microscope uses a Ti:sapphire light source (Mira 900, Coherent) with a center wavelength of 800 nm.…”

“…To overcome some of the limitations of SPRM sensors, we recently developed a SPP-based sensor that makes use of an alternative optical signal . Instead of monitoring small changes in the SPP-coupling angle or in the SPP interference pattern, this novel sensor uses the two-photon excited luminescence (TPEL) of the planar Au film as the sensing signal. − We demonstrated that the TPEL signal grows whenever a nanoscopic particle is within the nanoscale vicinity of the gold film, producing a localized emission source against a dark background that can be easily captured on a far-field camera.…”