Naveen Gandra

Plasmonic Planet–Satellite Analogues: Hierarchical Self-Assembly of Gold Nanostructures

In the past few years, a remarkable progress has been made in unveiling novel and unique optical properties of strongly coupled plasmonic nanostructures, known as plasmonic molecules. However, realization of such plasmonic molecules using nonlithographic approaches remains challenging largely due to the lack of facile and robust assembly methods. Previous attempts to achieve plasmonic nanoassemblies using molecular ligands were limited to dipolar assembly of nanostructures, which typically results in polydisperse linear and branched chains. Here, we demonstrate that core-satellite structures comprised of shape-controlled plasmonic nanostructures can be achieved through self-assembly using simple molecular cross-linkers. Prevention of self-conjugation and promotion of cross-conjugation among cores and satellites plays a key role in the formation of core-satellite heteroassemblies. The in-built electromagnetic hot-spots and Raman reporters of core-satellite structures make them excellent candidates for surface-enhanced Raman scattering probes.

Bilayered Raman‐Intense Gold Nanostructures with Hidden Tags (BRIGHTs) for High‐Resolution Bioimaging

Conventional SERS probes suffer from limited brightness and poor reproducibility and stability making them unsuitable for routine in vivo applications. A novel class of layered SERS probes is demonstrated in which individual nanostructures host electromagnetic hotspots, thus increasing brightness by more than two orders magnitude compared to conventional individual nanostructures.

Gold Nanorods as Plasmonic Nanotransducers: Distance-Dependent Refractive Index Sensitivity

Owing to the facile tunability of the localized surface plasmon resonance wavelength (LSPR) and large refractive index sensitivity, gold nanorods (AuNR) are of high interest as plasmonic nanotransducers for label-free biological sensing. We investigate the influence of gold nanorod dimensions on distance-dependent LSPR sensitivity and electromagnetic (EM) decay length using electrostatic layer-by-layer (LbL) assembly of polyelectrolytes. The electromagnetic decay length was found to increase linearly with both nanorod length and diameter, although to variable degrees. The rate of EM decay length increase with nanorod diameter is significantly higher compared to that of the length, indicating that diameter is a convenient handle to tune the EM decay length of gold nanorods. The ability to precisely measure the EM decay length of nanostructures enables the rational selection of plasmonic nanotransducer dimensions for the particular biosensing application.

Monitoring Controlled Release of Payload from Gold Nanocages Using Surface Enhanced Raman Scattering

Novel organic and inorganic nanostructures for localized and externally triggered delivery of therapeutic agents at a target site have received immense attention over the past decade owing to their enormous potential in treating complex diseases such as cancer. Gold nanocages, a novel class of hollow plasmonic nanostructures, have been recently demonstrated to serve as carriers for the delivery of payload with external trigger such as light or ultrasound. In this article, we demonstrate that surface enhanced Raman spectroscopy (SERS) can be employed to noninvasively monitor the release of payload from these hollow plasmonic nanostructures. The large enhancement of electromagnetic (EM) field at the interior surface of these nanostructures enables us to monitor the controlled release of Raman-active cargo from nanocages. Considering that SERS can be excited and collected in near-infrared (NIR) therapeutic window, this technique can serve as a powerful tool to monitor the drug release in vivo, providing additional control over externally triggered drug administration.

Bioplasmonic Paper as a Platform for Detection of Kidney Cancer Biomarkers

We demonstrate that a common laboratory filter paper uniformly adsorbed with biofunctionalized plasmonic nanostructures can serve as a highly sensitive transduction platform for rapid detection of trace bioanalytes in physiological fluids. In particular, we demonstrate that bioplasmonic paper enables rapid urinalysis for the detection of kidney cancer biomarkers in artificial urine down to a concentration of 10 ng/mL. Compared to conventional rigid substrates, bioplasmonic paper offers numerous advantages such as high specific surface area (resulting in large dynamic range), excellent wicking properties (naturally microfluidic), mechanical flexibility, compatibility with conventional printing approaches (enabling multiplexed detection and multimarker biochips), and significant cost reduction.

Multifunctional Hybrid Nanopatches of Graphene Oxide and Gold Nanostars for Ultraefficient Photothermal Cancer Therapy

Multifunctional hybrid nanomaterials with enhanced therapeutic efficiency at physiologically safe dosages for externally triggered, image-guided therapy are highly attractive for nanomedicine. Here, we demonstrate a novel class of multifunctional hybrid nanopatches comprised of graphene oxide (GO) and gold nanostars for enhanced photothermal effect and image-guided therapy. The hybrid nanopatches with tunable localized surface plasmon resonance into the near-infrared therapeutic window (650-900 nm) were realized using a biofriendly method that obviates the need for toxic shape-directing agents. Internalization of the intact nanopatches into epithelial breast cancer cells was confirmed by Raman imaging, transmission electron microscopy, and inductively coupled plasma mass spectrometry. It appears that the amphipathic nature and the large surface area of the graphene oxide enable it to serve as a soft, flexible, and biocompatible intracellular carrier for the in situ grown plasmonic nanostructures and provide long-term biocompatibility with extremely low cytotoxicity. Apart from a remarkably improved photothermal effect compared to that of either of the components at very low dosages of the hybrids (10 μg/mL GO) and using a low laser power (0.75 W cm(-2)), the hybrid nanopatches exhibit strong Raman scattering, making them excellent candidates for bioimaging, diagnostics, and image-guided therapy applications.

Probing Distance‐Dependent Plasmon‐Enhanced Near‐Infrared Fluorescence Using Polyelectrolyte Multilayers as Dielectric Spacers

Owing to their applications in biodetection and molecular bioimaging, near-infrared (NIR) fluorescent dyes are being extensively investigated. Most of the existing NIR dyes exhibit poor quantum yield, which hinders their translation to preclinical and clinical settings. Plasmonic nanostructures are known to act as tiny antennae for efficiently focusing the electromagnetic field into nanoscale volumes. The fluorescence emission from NIR dyes can be enhanced by more than thousand times by precisely placing them in proximity to gold nanorods. We have employed polyelectrolyte multilayers fabricated using layer-by-layer assembly as dielectric spacers for precisely tuning the distance between gold nanorods and NIR dyes. The aspect ratio of the gold nanorods was tuned to match the longitudinal localized surface plasmon resonance wavelength with the absorption maximum of the NIR dye to maximize the plasmonically enhanced fluorescence. The design criteria derived from this study lays the groundwork for ultrabright fluorescence bullets for in vitro and in vivo molecular bioimaging.

Sensitive DNA detection and SNP discrimination using ultrabright SERS nanorattles and magnetic beads for malaria diagnostics

One of the major obstacles to implement nucleic acid-based molecular diagnostics at the point-of-care (POC) and in resource-limited settings is the lack of sensitive and practical DNA detection methods that can be seamlessly integrated into portable platforms. Herein we present a sensitive yet simple DNA detection method using a surface-enhanced Raman scattering (SERS) nanoplatform: the ultrabright SERS nanorattle. The method, referred to as the nanorattle-based method, involves sandwich hybridization of magnetic beads that are loaded with capture probes, target sequences, and ultrabright SERS nanorattles that are loaded with reporter probes. Upon hybridization, a magnet was applied to concentrate the hybridization sandwiches at a detection spot for SERS measurements. The ultrabright SERS nanorattles, composed of a core and a shell with resonance Raman reporters loaded in the gap space between the core and the shell, serve as SERS tags for signal detection. Using this method, a specific DNA sequence of the malaria parasite Plasmodium falciparum could be detected with a detection limit of approximately 100 attomoles. Single nucleotide polymorphism (SNP) discrimination of wild type malaria DNA and mutant malaria DNA, which confers resistance to artemisinin drugs, was also demonstrated. These test models demonstrate the molecular diagnostic potential of the nanorattle-based method to both detect and genotype infectious pathogens. Furthermore, the methods simplicity makes it a suitable candidate for integration into portable platforms for POC and in resource-limited settings applications.

Biomimetic SERS substrate: peptide recognition elements for highly selective chemical detection in chemically complex media

Surface enhanced Raman scattering (SERS) is rapidly emerging as a sensitive transduction platform for the trace detection of chemical and biological analytes. A critical challenge that needs to be addressed to propel this technique into real world applications is the poor chemical selectivity of the existing SERS substrates. In this communication, we demonstrate a novel biomimetic approach to enhance the selectivity of plasmonic nanostructures to target chemical analytes. In particular, we demonstrate that material-binding peptides, identified through phage-display, serve as recognition elements for selective capture of target chemical species from a complex chemical mixture. As a proof of concept, we show that a nitroaromatic explosive molecule, trinitrotoluene (TNT), can be detected down to 100 pM concentration even in a complex organic chemical mixture. This ultrasensitive and selective detection is enabled by TNT-binding peptides appended to gold nanorods, which serve as selective SERS media. To the best of our knowledge, this is the first demonstration of a biomimetic SERS substrate facilitating selective and sensitive detection of a target chemical analyte in the presence of numerous unknown interfering species.

Gold nanocages with built-in artificial antibodies for label-free plasmonic biosensing

We demonstrate that gold nanocages (AuNCs) with built-in artificial antibodies enable the detection of kidney injury biomarker from synthetic urine down to a concentration of 25 ng ml-1. Molecularly imprinted AuNCs exhibit excellent selectivity against numerous interfering urinary proteins and remarkable stability over a wide range of pH and specific gravity.