Soft Nanomaterials Laboratory

Department of Mechanical Engineering & Materials Science


Soft Nanomaterials Lab: Research Focus

Novel plasmonic nanostructures and assemblies for theranostics: Owing to the unique combination of physical, chemical and biological properties such as large absorption and scattering cross-section, high sensitivity to local dielectric environment and enhanced electric field at the surface, metal nanostructures are emerging as an important class of materials for nanomedicine. However, there are several fundamental scientific challenges that need to be addressed before plasmonic materials can make an impact in clinical settings. We have specifically focused on surface enhanced Raman scattering (SERS) as a bioimaging modality for spectrum- or image-guided surgical tumor resection. One of the key challenges in this field is the limited brightness of the SERS probes that are employed as exogenous contrast agents for SERS-based bioimaging.  We addressed this issue by introducing several simple and robust chemical methods for achieving ultrabright SERS probes by either trapping Raman reporters between core-satellite assemblies of gold nanostructures or by sandwiching Raman reporters between gold core and shell of core-shell nanoparticles.  Furthermore, we have demonstrated the integration of such bright probes in drug delivery vehicles for image-guided locoregional cancer therapy.

    Figure 1: Schematic illustrations and TEM images of (A-C) core-satellite assemblies and (D-F) core-shell nanostructures

Printable multimarker biochips for point-of-care diagnostics
: Owing to their remarkable sensitivity to changes in the refractive index of surrounding medium, plasmonic nanostructures are considered to be an excellent choice for highly sensitive diagnostic devices.  We envision printable plasmonic biosensors for diagnostic applications in resource-limited and point-of-care settings. Towards this end, we have achieved two important breakthroughs in the last two years: (i) we have demonstrated an ultrasensive plasmonic biosensor based on ordinary filter paper adsorbed with biofunctionalized gold nanorods as nanotransducers.  The design obviates the need for complex micro and nanofabrication procedures to realize a highly sensitive biosensor.  Bioplasmonic paper, as we call it, enabled the detection of kidney cancer biomarkers in synthetic urine at clinically relevant concentrations.  In a more recent development, we have also demonstrated a multi-marker biochip using calligraphy, which involves the use of conventional ball point pen loaded with biofunctionalized nanostructures as ink, for the formation of discrete test domains on paper. (ii) We molecularly imprinted gold nanostructures using target proteins as templates to form a plasmonic biosensor based on artificial antibodies.  The imprinted plasmonic nanostructures with built-in recognition elements exhibited excellent sensitivity and selectivity, making them highly promising for point-of-care diagnostics.  Integration of these two important developments can lead to printable biochips, which will be the focus of future work in this direction.

Figure 2: (A) SEM image of biofunctionalized AuNR adsorbed on paper (B) Extinction spectra showing the shift in the LSPR spectra corresponding to the binding of the capture and target biomolecules on AuNR  Schematic illustration of (C) bioplamonic calligragraphy and (D-E) molecular imprinting on gold nanocages  (F) Plasmonic nanosensor response displaying high selectivity to target protein (NGAL).

Efficient SERS substrates for rapid chemical detection:
In the past four years, we have introduced numerous 2D and 3D SERS substrates based on vertically aligned ZnO nanowires, electrospun polymer fiber mats and common filter paper.  Owing to their flexible nature, paper-based SERS substrates could be employed as swabs for the collection and detection of trace levels (few nanograms) of chemical compounds spread on real-world surfaces.  We have also demonstrated a versatile approach that allows separation and preconcentration of different components of a complex sample in a small surface area by taking advantage of the properties of cellulose paper capillary effect.  As a proof-of-principle demonstration, a mixture of charged analytes were separated and concentrated at the sharp ends of the fingers of a star-shaped filter paper by functionalizing each finger with a different polyelectrolyte. Furthermore, the sharp ends of the fingers, served as pre-concentration sites for analytes deposited at the center of the star-shaped plasmonic paper substrates, owing to the shape-enhanced capillary effect.  A remarkable sub-attomolar detection of a model analyte (2-napathalenethiol) was noted at the micrometric detection spot i.e., sharp tips of the paper.  As a possible solution to the long-standing problem of poor selectivity of SERS substrates, we have introduced a generic biomimetic approach, which involves the integration of heterofunctional biological recognition elements (BRE) with plasmonic nanostructures to realize a chemically-selective SERS substrate.  As a proof-of-principle, we employed a BRE peptide (Cys-(Gly)11-Trp-His-Trp-Gln-Arg-Pro-Leu-Met-Pro-Val-Ser-Ile) comprised of trinitrotoluene (TNT)-binding moiety (underlined) and a cysteine-terminal that enables strong and stable binding of peptide to AuNR through Au-S bond. The glycine spacer in between these two moieties provides conformational flexibility to the TNT-binding moiety by avoiding steric hindrance.  This approach can be easily extended to other chemical and biological analytes taking SERS closer to real-world chemical sensing.  

Figure 3: Photograph showing the paper-based SERS substrate being swabbed a glass surface to collect trace amounts of analyte (B) SEM image showing the paper fibers uniformly decorated with gold nanorods (C) Photograph of the star-shaped plasmonic paper. Zoom-in shows the dark-field image of the tip indicating the areas from which SERS spectra shown in (D) were collected. (E) Schematic showing the biomimetic SERS substrate.

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