Rakesh Singh Moirangthem

Enhanced localized plasmonic detections using partially-embedded gold nanoparticles and ellipsometric measurements

A cost-effective, stable and ultrasensitive localized surface plasmon resonance (LSPR) sensor based on gold nanoparticles (AuNPs) partially embedded in transparent substrate is presented. Partially embedded AuNPs were prepared by thermal annealing of gold thin films deposited on glass at a temperature close to the glass transition temperature of the substrate. Annealed samples were optically characterized by using spectroscopic ellipsometry and compare with theoretical modeling to understand the optical responses from the samples. By combining the partially-embedded AuNPs substrate with a microfluidic flow cell and dove prism in an ellipsometry setup, an ultrasensitive change in the LSPR signal can be detected. The refractive index sensitivity obtained from the phase measurement is up to 1938 degrees/RIU which is several times higher than that of synthesized colloidal gold nanoparticles. The sample is further used to investigate the interactions between primary and secondary antibodies. The bio-molecular detection limit of the LSPR signal is down to 20 pM. Our proposed sensor is label free, non-destructive, with high sensitivity, low cost, and easy to fabricate. These features make it feasible for commercialization in biomedical applications.

Optical cavity modes of a single crystalline zinc oxide microsphere

A detailed study on the optical cavity modes of zinc oxide microspheres under the optical excitation is presented. The zinc oxide microspheres with diameters ranging from 1.5 to 3.0 µm are prepared using hydrothermal growth technique. The photoluminescence measurement of a single microsphere shows prominent resonances of whispering gallery modes at room temperature. The experimentally observed whispering gallery modes in the photoluminescence spectrum are compared with theoretical calculations using analytical and finite element methods in order to clarify resonance properties of these modes. The comparison between theoretical analysis and experiment suggests that the dielectric constant of the ZnO microsphere is somewhat different from that for bulk ZnO. The sharp resonances of whispering gallery modes in zinc oxide microspheres cover the entire visible window. They may be utilized in realizations of optical resonators, light emitting devices, and lasers for future chip integrations with micro/nano optoelectronic circuits, and developments of optical biosensors.

Surface plasmon resonance ellipsometry based sensor for studying biomolecular interaction.

A simple surface plasmon resonance (SPR) ellipsometry equipped with a dove prism and micro-fluidic flow cell is adopted to investigate and study basic properties of biomolecular interaction. Using a dove prism greatly simplifies the optical alignment and the use of micro-fluidic cell helps reduce significantly the volume of the biological sample required in the experiment. By recording the ellipsometry data in terms of relative changes in the ellipsometric parameters, Psi and Delta as sensor signals we can understand the biomolecular interaction. Spectroscopic measurements were performed to check the bulk sensitivity, which further helps determine the corresponding wavelength for maximum sensitivity. Furthermore, dynamic measurements at a fixed wavelength were also done and allow the observation of real-time response to the changes in surface properties on a metallic film. Such a simple technique gives an index resolution around 1.64x10(-6) which is better than the conventional SPR method based on the resonance angular detection. This technique yields sensitivity sufficient enough to detect changes in the effective thickness of biomolecular layer. Biological processes such as adsorption of protein to metal and protein-protein interaction can be understood from the optical response of the sample surface. Such technique is a promising candidate in developing profitable and user-friendly biosensors. Furthermore, this kind of characterization technique is non-destructive, label free, and sensitive with a sub-nanometer resolution in thickness.

Investigation of surface plasmon biosensing using gold nanoparticles enhanced ellipsometry

We present a label-free, nondestructive and high sensitivity biosensor by using the phase information of a gold nanoparticles enhanced ellipsometry signal. The refractive index (RI) resolution from ellipsometric phase information is of the order of 1.6×10(-6) RI units. Furthermore, spectroscopic and dynamic measurements show substantial change in the phase signal when biomolecules are coated on gold nanoparticles. The detection limit of our proposed technique is up to ∼18 pM concentration of the target biomolecules.

Interfacial refractive index sensing using visible-excited intrinsic zinc oxide photoluminescence coupled to whispering gallery modes

Whispering gallery modes (WGMs) excited by the intrinsic photoluminescence (PL) in zinc oxide microspherical resonators are investigated in this work. The microspheres were synthesized via a one-pot hydrothermal technique. A polymer was applied after the synthesis to fill remaining pores in the oxide particle. Defect-related ZnO PL was excited in the visible, coupling to WGMs. The observed WGMs red-shift with increasing refractive index of the surrounding medium with a sensitivity of 90–100 nm/refractive index unit. The spherical microresonators may be used to investigate binding to and structure at the particle/solution interface.

Ellipsometry study on gold-nanoparticle-coated gold thin film for biosensing application

The amplified plasmonic response from various distributions of gold nanoparticles (AuNPs) coated on top of gold thin film was studied via ellipsometry under total internal reflection mode. The surface plasmon resonance dip can be tuned from the visible to near infrared by simply varying the AuNP concentration. Theoretical modeling based on effective medium theory with a multi-slice model has been employed to fit the experimental results. Additionally, this experimental tool has been further extended to study bio-molecular interactions with metal surfaces as well as in studying protein-protein interaction without any labeling. Hence, this technique could provide a non-destructive way of designing tunable label-free optical biosensors with very high sensitivity.

Local plasmonic resonance based biosensor for investigating DNA hybridization using ellipsometry

A label free, non-destructive and high sensitivity biosensor with sub-nanometer thickness resolution is presented. We investigate various sequences of DNA attached on gold nanoparticles (AuNPs) on top of a layer of self assembling molecules. A strategy to amplify localized surface plasmon resonance (LSPR) response is made by sandwiching DNA sequences between two AuNPs. We monitor the induced changes in polarization state or phase of reflected light from the surface as a function of the photon energy as a sensor signal by using ellipsometry and compare that with theoretical simulation.

Biosensing via light scattering from plasmonic core-shell nanospheres coated with DNA molecules

We present both experimental and theoretical studies for investigating DNA molecules attached on metallic nanospheres. We have developed an efficient and accurate numerical method to investigate light scattering from plasmonic nanospheres on a substrate covered by a shell, based on the Green’s function approach with suitable spherical harmonic basis. Next, we use this method to study optical scattering from DNA molecules attached to metallic nanoparticles placed on a substrate and compare with experimental results. We obtain fairly good agreement between theoretical predictions and the measured ellipsometric spectra. The metallic nanoparticles were used to detect the binding with DNA molecules in a microfluidic setup via spectroscopic ellipsometry (SE), and a detectable change in ellipsometric spectra was found when DNA molecules are captured on Au nanoparticles. Our theoretical simulation indicates that the coverage of Au nanosphere by a submonolayer of DNA molecules, which is modeled by a thin layer of dielectric material (which may absorb light), can lead to a small but detectable spectroscopic shift in both the Ψ and Δ spectra with more significant change in Δ spectra in agreement with experimental results. Our studies demonstrated the ultrasensitive capability of SE for sensing submonolayer coverage of DNA molecules on Au nanospheres. Hence the spectroscopic ellipsometric measurements coupled with theoretical analysis via an efficient computation method can be an effective tool for detecting DNA molecules attached on Au nanoparticles, thus achieving label-free, non-destructive, and high-sensitivity biosensing with nanoscale resolution.

Huge SERS enhancement via ZnO nanowires on gold nanoislands

We experimentally investigated Zinc oxide (ZnO) nanowires (NWs) on flat Si substrate and ZnO NWs on Au nanoislands attached on a Si substrate via hydrothermal technique, pursuing surface enhanced Raman scattering (SERS). Au nanoislands were formed by thermal annealing of a Au thin film deposited on Si substrate. ZnO NWs were then grown on two types of substrates (with and without Au nanoislands) and thermally annealed together. During the thermal annealing process, the ZnO NWs were coupled to Au nanoislands. After the thermal annealing, strong SERS enhancement was observed of ZnO NWs on Au nanoislands. Over 30 times enhancement in SERS was found when the initial Au layer thickness was 40 nm.

Plasmonic biosensing with nanoimprint binary grating using ellipsometry

An optical label free and high sensitivity plasmonic biosensor using nanoimprint metallic binary grating is presented based on the phase information of the ellipsometry signal. Plasmonic binary grating was prepared by using soft nanoimprinting technique which significantly reduce the fabrication cost and can be realized for a transition from a laboratory-scale method to full-scale technology. The bulk sensitivity measurement from this 1D binary metallic grating gives a value of refractive index resolution of 1.06×10-7 RIU. Such a highly sensitive plasmonic biochip was used to investigate the adsorption of bio-molecules on the nanostructure surface in dynamic mode by monitoring the change in polarization state or phase of reflected light in the ellipsometry measurement as a sensing signal.