Venkata Ramanaiah Dantham

Label-free detection of single protein using a nanoplasmonic-photonic hybrid microcavity.

Recently we reported the detection and sizing of the smallest RNA virus MS2 with a mass of 6 ag from the resonance frequency shift of a whispering gallery mode-nanoshell hybrid resonator (WGM-h) upon adsorption on the nanoshell and anticipated that single protein above 0.4 ag should be detectable but with considerably smaller signals. Here, we report the detection of single thyroid cancer marker (Thyroglobulin, Tg) and bovine serum albumin (BSA) proteins with masses of only 1 ag and 0.11 ag (66 kDa), respectively. However, the wavelength shifts are enhanced beyond those anticipated in our earlier work by 240% for Tg and 1500% for BSA. This surprising sensitivity is traced to a short-range reactive field near the surface of our Au nanoshell receptor due to intrinsic random bumps of protein size, leading to an unanticipated increase in sensitivity to single protein, which grows larger as the protein diminishes in size. As a consequence of the largest signal-to-noise ratio in our BSA experiments (S/N ≈ 13), we conservatively estimated a new protein limit of detection for our WGM-h of 5 kDa.

Taking whispering gallery-mode single virus detection and sizing to the limit

We report the label-free detection and sizing by a microcavity of the smallest individual RNA virus, MS2, with a mass only ∼1% of InfluenzaA (6 vs. 512 ag). Although detection of such a small bio-nano-particle has been beyond the reach of a bare spherical microcavity, it was accomplished with ease (S/N = 8, Q = 4 × 105) using a single dipole stimulated plasmonic-nanoshell as a microcavity wavelength shift enhancer, providing an enhancement of ∼70×, in agreement with theory. Unique wavelength shift statistics are recorded consistent with an ultra-uniform genetically programmed substance that is drawn to the plasmonic hot spots by light-forces.

Enhancement of Raman scattering by two orders of magnitude using photonic nanojet of a microsphere

The enhancement of Raman signal is observed on excitation through a single microsphere. The dependence of the enhancement ratio (ER) on various parameters viz., numerical aperture (NA) of the microscopic objective lens, pump wavelength, size and refractive index of the microsphere has been studied. The enhancement has been explained due to interaction of the increased field of the photonic nanojet emerging from the single microsphere. The photonic nanojet induced ER of Raman peaks of silicon wafer and cadmium ditelluride is reported here. It is observed for the first time that by suitable selection of the experimental parameters, it is possible to enhance the Raman signal by approximately two orders of magnitude.

Periodic plasmonic enhancing epitopes on a whispering gallery mode biosensor

We propose the attachment of a periodic array of gold nanoparticles (epitopes) to the equator of a Whispering Gallery Mode Biosensor for the purpose of plasmonically enhancing nanoparticle sensing in a self-referencing manner while increasing the capture rate of analyte to antibodies attached to these plasmonic epitopes. Our approach can be applied to a variety of whispering gallery mode resonators from silicon/silica rings and disks to capillaries. The interpretation of the signals is particularly simple since the optical phase difference between the epitopes is designed to be an integer multiple of ?, allowing the wavelength shift from each binding event to add independently.

Polynomial Smoothing of Time Series With Additive Step Discontinuities

This paper addresses the problem of estimating simultaneously a local polynomial signal and an approximately piecewise constant signal from a noisy additive mixture. The approach developed in this paper synthesizes the total variation filter and least-square polynomial signal smoothing into a unified problem formulation. The method is based on formulating an l1-norm regularized inverse problem. A computationally efficient algorithm, based on variable splitting and the alternating direction method of multipliers (ADMM), is presented. Algorithms are derived for both unconstrained and constrained formulations. The method is illustrated on experimental data involving the detection of nano-particles with applications to real-time virus detection using a whispering-gallery mode detector.

High-Q whispering gallery modes of doped and coated single microspheres and their effect on radiative rate

Fluorescence spectra of two organic dyes doped in polymer beads as well as coated on single microparticles of silica exhibit whispering gallery modes (WGMs). For doped microspheres, theoretical simulations on WGMs have been carried out based on the Lorentz-Mie theory by varying the refractive index and the diameter of the microparticle. Similarly, for a coated microsphere, an Aden and Kerker model of the Lorentz-Mie theory has been used to simulate WGMs. For diameters ⩾8 μm, low-resolution simulations of scattering efficiency fail to show modes of higher-quality factors (Q~108). A new procedure of identifying these modes is given here that does not require use of high-performance computing. Effects of WGMs on decay rates have also been studied. It has been found that, while doped microparticles exhibit no effect on the radiative rate, coated microparticles show inhibition of the decay rate for smaller sizes. Decay rates of single-coated microspheres are found to be distinctly different from those of randomly shaped single microcrystals.

Label-free single cancer marker protein detection using a nanoplasmonic-photonic hybrid whispering gallery mode biosensor

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.

Numerical investigations on photonic nanojet mediated surface enhanced Raman scattering and fluorescence techniques

Finite element method simulations have been carried out on the photonic nanojet (PNJ) mediated surface enhanced Raman scattering (SERS) technique for the first time, and this technique has been found to provide (i) better Raman scattering enhancement of single molecules and (ii) a long working distance between the microscopic objective lens and sample, as compared with the conventional SERS technique. A PNJ mediated surface enhanced fluorescence (SEF) technique has been proposed to enhance the fluorescence of single molecules using the combination of localized surface plasmons inside nanostructures and the PNJ of a dielectric microsphere (MS), and this technique is numerically proved to be efficient as compared with a conventional SEF technique. Moreover, the generation of a PNJ from single lollipop shaped microstructures and its applications in the above mentioned techniques have been reported.

Efficient nanoplasmonic antennas for fabricating single protein molecule detector

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.

Microcavity single virus detection and sizing with molecular sensitivity

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, Q = 4x105) 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].