Biosensors with high sensitivity are sought for a range of diagnostic applications, including detection of biomarkers at ultralow levels detection of disease risk, diagnosis or prognosis and as research tools to investigate questions of clinical relevance, e.g. profiling protein expression in single cell assays. Typical detection limits of bioassays in the market are in the nanomolar concentrations,1 while the emerging demands require sensitivity to be better by at least three orders of magnitude, viz. pico to the femtomolar regime. Achieving such high sensitivity is a significant challenge, where nanoplasmonic sensing considerable high promise.
Nanoplasmonic sensing relies on giant electromagnetic field enhancements arising at the immediate vicinity of metal nanostructures upon excitation of localized surface plasmon polaritons by light irradiation at a suitable wavelength.2 For noble metals such as gold or silver EM field enhancements as high as 109-1012 can be realized in spatially confined volumes, defined by nanoscale gaps or curvatures, typically in the sub-10nm regime. Molecules that come in close vicinity to these regions of high EM enhancements (also known as hot-spots) can be detected via their vibrational Raman (surface-enhanced Raman scattering, or SERS) or fluorescence (metal-enhanced Fluorescence, or MEF) signals with detection limits potentially down to single molecule level. However, these confined volumes impose severe spatial constraints in accommodating biomolecular binding events, thus making it particularly challenging to take advantage of the high EM enhancements at the hot-spots. To this end, my thesis aims at rational design of nanoscale geometries that can enable co-localization of the reporter of biomolecular binding events with the plasmonic hot-spots to realize highly sensitive plasmonic biosensors based on surface-enhanced Raman spectroscopy.
Scientific and Technical Challenges
The key challenge for the thesis is to identify hot-spot configurations that present high EM enhancements which also allows sufficient space to accommodate biomolecular binding events. This requires both an ability to create and interrogate hot-spots, and controlling biomolecular events at the nanoscale.
I. Engineered hot-spots
a. High geometry control: The fabrication process to create hot-spots should deliver a high level of control over hot-spot geometries, with geometric attributes that are consistent across the sample and reproducible across batches, preferably within a tolerance of 10%. This is necessary to achieve high enhancement factors, with low signal intensity variations to facilitate rational design through modelling and simulations and to avoid false negatives in the eventual biosensor.
b. High density of hot-spots spanning large area: This will facilitate higher signal levels, and also allow larger dynamic range for sensing. The larger area would enable use of probing tools/techniques with a macroscopic footprint (of several microns to square millimetres). Achieving this may impose a heavy penalty of cost or time using most commonly available nanofabrication tools that deliver high control (e.g. e-beam lithography, focused ion beam lithography)
c. Amenable to rational design: Rational design requires systematic investigations requiring correlations between process parameters, nanostructure geometry, optical/spectroscopic property. This requires a geometry that can be readily modelled, with a minimal deviation of such model from experimentally realized geometries.
d. The hot-spot geometry should take into account the length-scales of the biomolecular binding event for the bioassay in question. For a typical immunoassay, the binding event can measure up to 40 nm.3
e. The characterization of the geometry of hot-spot is not always straightforward. The solution has relied on building an understanding based on information acquired from multiple techniques, e.g. SEM, AFM, geometric model, optical simulations.
II. Biomolecular binding events at confined spaces
a. Attachment of biomolecules: Attempts to position biomolecules at hot-spots using typical strategies employed for patterning biomolecules would require selective surface functionalization at length scales of only a few nanometers. This imposes the severe challenge to preserving the integrity of the final functionalized surface.
b. Understanding the nano-bio interface: Nanostructures on the surface are known to influence either favourably or adversely the interaction of biomolecules with surface.4 Such interaction can be tailored by controlling the environment (e.g. pH, the ionic strength of the medium, the zeta potential on the surface, etc.). It is a challenge to coordinate the fabrication to ensure that the plasmonic interface can be subjected to the biosensing experiments and the multiple changes to the biosensing environment without losing stability.
Rishabh Rastogi, Suresh Poovanthingal, Pierre Michel Adam, Jewgeni Starikov, Saulius Juodkazis, and Sivashankar Krishnamoorthy, "Engineering nanopattern geometry at molecular resolution to enhance sensitivity of nanoplasmonic biosensors (Conference Presentation)," Proc. SPIE 10685, Biophotonics: Photonic Solutions for Better Health Care VI, 106850U (Presented at SPIE Photonics Europe: April 24, 2018; Published: 24 May 2018); https://doi.org/10.1117/12.2306809.5789228914001.
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