Rapid Detection of Aβ Aggregation and Inhibition by Dual Functions of Gold Nanoplasmic Particles: Catalytic Activator and Optical Reporter

One of the primary pathological hallmarks of Alzheimer’s diseases (AD) is amyloid-β (Aβ) aggregation and its extracellular accumulation. However, current <i>in vitro</i> Aβ aggregation assays require time-consuming and labor-intensive steps, which delay the process of drug discovery and understanding the mechanism of Aβ induced neurotoxicity. Here, we propose a rapid detection method for studying Aβ aggregation and inhibition under an optimized acidic perturbation condition by dual functions of gold nanoplasmonic particles (GNPs): (1) catalytic activator and (2) optical reporter. Because of roles of GNPs as effective nucleation sites for fast-catalyzing Aβ aggregation and colorimetric optical reporters for tracking Aβ aggregation, we accomplished the fast aggregation assay in less than 1 min by the naked eyes. Our detection method is based on spontaneous clustering of unconjugated (unmodified) GNPs along with the aggregated Aβ network under an aggregation-promoting condition. As a proof-of-concept demonstration, we employed the acidic perturbation permitting rapid cooperative assemblies of GNPs and Aβ peptides <i>via</i> their surface charge modulation. Under the optimized acidic perturbation condition around pH 2 to 3, we characterized the concentration-dependent colorimetric responses for aggregation at physiologically relevant Aβ concentration levels (from 100 μM to 10 nM). We also demonstrated the GNP/acidic condition-based rapid inhibition assay of Aβ aggregation by using well-known binding reagents such as antibody and serum albumin. The proposed methodology can be a powerful alternative method for screening drugs for AD as well as studying molecular biophysics of protein aggregations, and further extended to explore other protein conformational diseases such as neurodegenerative disease

Facile Amplification of Solution-State Surface-Enhanced Raman Scattering of Small Molecules Using Spontaneously Formed 3D Nanoplasmonic Wells

Surface-enhanced Raman scattering (SERS) has recently been considered as one of the most promising tools to directly analyze small molecules without labels, owing to advantages in sensitivity, specificity, and speed. However, collecting reproducible SERS signals from small molecules on substrates or in solutions is challenging because of random molecular adsorption on surfaces and laser-induced molecular convection in solutions. Herein, we report a novel and efficient way to collect SERS signals from solution samples using three-dimensional nanoplasmonic wells spontaneously formed by interfacial reactions between liquid polydimethylsiloxane (PDMS) and small droplets of metal ion solutions (e.g., HAuCl₄ and AgNO₃). A SERS signal is easily maximized at the center near the bottom of the well due to spherical feature of the fabricated wells and electromagnetic field enhancement by the metallic nanoparticles (e.g., Au and Ag) integrated on their surfaces. Through the systematic control over the volume, concentration, and composition of the metal ion solution, optical functions of the nanoplasmonic wells were optimized for SERS, which was further amplified by exploiting the plasmonic couplings with colloidal nanoparticles. By using the optimized nanoplasmonic wells and the detection protocol, we successfully obtained intrinsic spectra of biomolecules (e.g., adenine, glucose, amyloid β) and toxic environmental molecules (e.g., 1,1′-diethyl-2,2′-cyanine iodide and chloromethyliothiazolinone/methylisothiazolinone) as well as Raman active molecules, such as rhodamine 6G and 1,2-bis­(4-pyridyl)­ethylene at a low concentrations down to the picomolar level. Our detection platform provides a powerful way to develop highly sensitive sensors and high-throughput analyzing protocols for fieldwork applications as well as diagnosing diseases

Gold Nanoparticles as Nucleation-Inducing Reagents for Protein Crystallization

Protein crystallization is a necessary but time-consuming and difficult step in structure determination by crystallography. The rate-limiting step of crystallization is nucleation, where protein monomers in solution cluster in an ordered fashion to form a stable nucleus. Here, we propose the use of gold nanoparticles to accelerate nucleation and enhance crystal formation. We tested whether gold nanoparticles can facilitate nucleation by interacting with target proteins and inducing favorable clustering. We used differently sized gold nanospheres and gold nanostars to crystallize hen egg-white lysozyme. Our results indicated that gold nanoparticles significantly increased the number of crystallization conditions (by about 20%). Spherical and larger gold particles were found to be more efficient. Furthermore, the use of gold nanoparticles did not have any adverse effect on data collection or structure determination. Our findings indicate that the use of nanoparticles as protein nucleation-inducing reagents can greatly accelerate structure determination by X-ray crystallography

On-Chip Colorimetric Detection of Cu²⁺ Ions via Density-Controlled Plasmonic Core–Satellites Nanoassembly

We report on an on-chip colorimetric method for the detection and analysis of Cu²⁺ ions via the targeted assembly of plasmonic silver nanoparticles (2.6 nm satellites) on density-controlled plasmonic gold nanoparticles (50 nm cores) on a glass substrate. Without any ligand modification of the nanoparticles, by directly using an intrinsic moiety (carboxylate ion, COO^–) surrounded with nanoparticles, the method showed a high selectivity for Cu²⁺, resulting in a nearly 2 times greater optical response compared to those of other metal ions via the targeted core–satellites assembly. By modulating the surface chemistry, it was possible to control the density of core gold nanoparticles on the surface, thus permitting easy tuning of the optical responses induced by plasmon coupling generated between each core–satellites nanostructure. Using chips with a controlled optimal core density, we observed the remarkable scattering color changes of the chips from green to yellow and finally to orange with the increase of Cu²⁺ concentration. The detection limits of the fabricated chips with controlled core densities (ca. 1821 and 3636 particles/100 μm²) are 10 nM and 10 pM, respectively, which are quite tunable and below the level of 20 μM (or 1.3 ppm) defined by the United States Environmental Protection Agency. The findings suggest that the method is a potentially promising protocol for detecting small molecules with target selectivity and the tunability of the detection limits by replacing with ligands and adjusting core densities

Core–Satellites Assembly of Silver Nanoparticles on a Single Gold Nanoparticle via Metal Ion-Mediated Complex

We report core–satellites (Au–Ag) coupled plasmonic nanoassemblies based on bottom-up, high-density assembly of molecular-scale silver nanoparticles on a single gold nanoparticle surface, and demonstrate direct observation and quantification of enhanced plasmon coupling (i.e., intensity amplification and apparent spectra shift) in a single particle level. We also explore metal ion sensing capability based on our coupled plasmonic core–satellites, which enabled at least 1000 times better detection limit as compared to that of a single plasmonic nanoparticle. Our results demonstrate and suggest substantial promise for the development of coupled plasmonic nanostructures for ultrasensitive detection of various biological and chemical analytes

Integrated Microalgae Analysis Photobioreactor for Rapid Strain Selection

Algal photosynthesis is considered to be a sustainable, alternative, and renewable solution to generating green energy. For high-productivity algaculture in diverse local environments, a high-throughput screening method is needed to select algal strains from naturally available or genetically engineered strains. Herein, we present an integrated plasmonic photobioreactor for rapid, high-throughput screening of microalgae. Our 3D nanoplasmonic optical cavity-based photobioreactor permits the amplification of a selective wavelength favorable to photosynthesis in the cavity. The hemispheric plasmonic cavity allows intercellular interaction to be promoted in the optically favorable milieu and also permits effective visual examination of algal growth. Using Chlamydomonas reinhardtii, we demonstrated a 2-fold enhanced growth rate and a 1.5-fold lipid production rate with no distinctive lag phase. By facilitating growth and biomass conversion rates, the integrated microalgae analysis platform will serve as rapid microalgae screening platforms for biofuel applications

Tunable Plasmonic Cavity for Label-free Detection of Small Molecules

Owing to its high sensitivity and high selectivity along with rapid response time, plasmonic detection has gained considerable interest in a wide variety of sensing applications. To improve the fieldwork applicability and reliability of plasmonic detection, the integration of plasmonic nanoparticles into optical devices is desirable. Herein, we propose an integrated label-free detection platform comprising a plasmonic cavity that allows sensitive molecular detection via either surface-enhanced Raman scattering (SERS) or plasmon resonance energy transfer (PRET). A small droplet of metal ion solution spontaneously produces a plasmonic cavity on the surface of uncured poly­(dimethylsiloxane) (PDMS), and as PDMS is cured, the metal ions are reduced to form a plasmonic antennae array on the cavity surface. Unique spherical feature and the integrated metallic nanoparticles of the cavity provide excellent optical functions to focus the incident light in the cavity and to rescatter the light absorbed by the nanoparticles. The optical properties of the plasmonic cavity for SERS or PRET are optimized by controlling the composition, size, and density of the metal nanoparticles. By using the cavity, we accomplish both 1000-fold sensitive detection and real-time monitoring of reactive oxygen species secreted by live cells via PRET. In addition, we achieve sensitive detection of trace amounts of toxic environmental molecules such as 5-chloro-2-methyl-4-isothiazolin-3-one/2-methyl-4-isothiazol-3-one (CMIT/MIT) and bisphenol A, as well as several small biomolecules such as glucose, adenine, and tryptophan, via SERS

Quantum Electrodynamic Behavior of Chlorophyll in a Plasmonic Nanocavity

Plasmonic nanocavities have been used as a novel platform for studying strong light–matter coupling, opening access to quantum chemistry, material science, and enhanced sensing. However, the biomolecular study of cavity quantum electrodynamics (QED) is lacking. Here, we report the quantum electrodynamic behavior of chlorophyll-a in a plasmonic nanocavity. We construct an extreme plasmonic nanocavity using Au nanocages with various linker molecules and Au mirrors to obtain a strong coupling regime. Plasmon resonance energy transfer (PRET)-based hyperspectral imaging is applied to study the electrodynamic behaviors of chlorophyll-a in the nanocavity. Furthermore, we observe the energy level splitting of chlorophyll-a, similar to the cavity QED effects due to the light–matter interactions in the cavity. Our study will provide insight for further studies in quantum biological electron or energy transfer, electrodynamics, the electron transport chain of mitochondria, and energy harvesting, sensing, and conversion in both biological and biophysical systems

Spontaneous Self-Formation of 3D Plasmonic Optical Structures

Self-formation of colloidal oil droplets in water or water droplets in oil not only has been regarded as fascinating fundamental science but also has been utilized in an enormous number of applications in everyday life. However, the creation of three-dimensional (3D) architectures by a liquid droplet and an immiscible liquid interface has been less investigated than other applications. Here, we report interfacial energy-driven spontaneous self-formation of a 3D plasmonic optical structure at room temperature without an external force. Based on the densities and interfacial energies of two liquids, we simulated the spontaneous formation of a plasmonic optical structure when a water droplet containing metal ions meets an immiscible liquid polydimethylsiloxane (PDMS) interface. At the interface, the metal ions in the droplet are automatically reduced to form an interfacial plasmonic layer as the liquid PDMS cures. The self-formation of both an optical cavity and integrated plasmonic nanostructure significantly enhances the fluorescence by a magnitude of 1000. Our findings will have a huge impact on the development of various photonic and plasmonic materials as well as metamaterials and devices

Three-Dimensional Reduced-Symmetry of Colloidal Plasmonic Nanoparticles

Owing to their novel optical properties, three-dimensional plasmonic nanostructures with reduced symmetry such as a nanocrescent and a nanocup have attracted considerable current interest in biophotonic imaging and sensing. However, their practical applications have been still limited since the colloidal synthesis of such structures that allows, in principle, for in vivo application and large-scale production has not been explored yet. To date, these structures have been fabricated only on two-dimensional substrates using micro/nanofabrication techniques. Here we demonstrate an innovative way of breaking symmetry of colloidal plasmonic nanoparticles. Our strategy exploits the direct overgrowth of Au on a hybrid colloidal dimer consisting of Au and polystyrene (PS) nanoparticles without the self-nucleation of Au in an aqueous solution. Upon the overgrowth reaction, the steric crowding of PS leads to morphological evolution of the Au part in the dimer ranging from half-shell, nanocrescent to nanoshell associated with the appearance of the second plasmon absorption band in near IR. Surface-enhanced Raman scattering signal is obtained directly from the symmetry-broken nanoparticles solution as an example showing the viability of the present approach. We believe our concept represents an important step toward a wide range of biophotonic applications for optical nanoplasmonics such as targeting, sensing/imaging, gene delivery, and optical gene regulations