Real time (label-free) detection and sizing of single protein molecule at its natural state is “holy grail” in biosensing
field. This non-destructive technique is useful for predicting the dangerous diseases at very early-stage. Herein, we
report the synthesis and characterization of efficient nanoplasmonic antennas, which could be useful to fabricate an
ultrasensitive nanoplasmonic-whispering gallery mode hybrid microresonator for the real time detection and sizing of
single protein molecule. This hybrid microresonator could be easily converted as an ultrasensitive single molecule
biosensor by anchoring suitable anti-bodies on the surface of the plasmonic nanoantenna.
The whispering gallery mode (WGM) biosensor is a micro-optical platform capable of sensitive label-free detection of
biological particles. Described by the reactive sensing principle (RSP), this analytic formulation quantifies the response
of the system to the adsorption of bioparticles. Guided by the RSP, the WGM biosensor enabling from detection of virus
(e.g., Human Papillomavirus, HPV) to the ultimate goal of single protein detection. The latter was derived from insights
into the RSP, which resulted in the development of a hybrid plasmonic WGM biosensor, which has recently
demonstrated detection of individual protein cancer markers. Enhancements from bound gold nanoparticles provide the
sensitivity to detect single protein molecules (66 kDa) with good signal-to-noise (S/N > 10), and project that detection of
proteins as small as 5 kDa.
The reactive sensing principle applied to hybrid plasmonic whispering gallery mode biosensor has recently demonstrated detection of individual protein cancer markers. The rough surface of a gold nanoparticle affixed to the resonator surface acts like a nanoscopic antenna, significantly boosts the local electric field within the cavity mode. Adsorption of a target protein onto this nanoscopic antenna results in an enhanced response of the resonator system to the binding event. We have demonstrated detection of individual protein molecules (66 kDa) with good signal-to-noise (S/N > 10), and project that detection of proteins as small as 5 kDa are possible.
Our hybrid plasmonic whispering gallery mode biosensor has recently demonstrated detection and characterization of the smallest known RNA virus. A gold nanoparticle affixed to the resonator surface acts like a nanoscopic antenna, enhancing locally the electric field within the cavity mode. When a target analyte binds with this nanoscopic antenna the result is an enhanced response (spectral shift) of the resonator system to the binding event. We have observed shift enhancements ~70× over the response of the bare resonator, thereby permitting the detection and characterization of all known viral particles and even some large protein molecules.
We report the label-free detection and sizing of the smallest individual RNA virus, MS2 by a spherical microcavity. Mass of this virus is ~6 ag and produces a theoretical resonance shift ~0.25 fm upon adsorbing an individual virus at the
equator of the bare microcavity, which is well below the r.m.s background noise of 2 fm. However, detection was
accomplished with ease (S/N = 8, <i>Q</i> = 4x10<sup>5</sup>) using a single dipole stimulated plasmonic-nanoshell as a microcavity wavelength shift enhancer. Analytical expressions based on the “reactive sensing principle” are developed to extract the radius of the virus from the measured signals. Estimated limit of detection for these experiments was ~0.4 ag or 240 kDa below the size of all known viruses, largest globular and elongated proteins [Phosphofructokinase (345 kDa) and Fibrinogen (390 kDa), respectively].