Synthesis of Ruby Red Colloidal Gold: Nanotechnology Demonstration

Gold nanoparticles enable simple diagnostic tests like pregnancy detection, where hCG protein prevents aggregation in pregnant women's urine, maintaining red color, while non-pregnant urine causes aggregation and color change. Protein-mediated stabilization depends on protein structure and charge determining surface affinity. For selective detection, nanoparticles are functionalized with antibodies, DNA, or other binding molecules. Surface modification uses polyethyleneimine (PEI) for covalent attachment of biomolecules. Color enhancement via silver ion deposition changes red to black, improving sensitivity. One-pot synthesis combines gold-binding proteins, antibodies, and gold ions in single steps. Click chemistry enables functionalization by attaching long-chain polymers with alkene groups to sample holders containing antigens, with gold nanoparticles modified with azide groups undergoing cycloaddition reactions.

Attaching biomolecules to gold nanoparticles requires choosing between passive adsorption and covalent conjugation. Passive binding relies on electrostatic interactions but requires optimization of pH and salt conditions with risk of aggregation. Covalent conjugation using overcoat technology eliminates optimization needs and provides greater stability. Oriented coupling strategies (malamide, hydrazide surfaces) position antibody binding sites outward for improved accessibility and signal intensity compared to random lysine coupling. These strategies enable development of sensitive assays with consistent performance across batches.

Gold nanoparticles are synthesized in aqueous solutions at room temperature using green chemistry approaches, featuring a surfactant bilayer with quaternary ammonium head groups creating positive charge and hydrophobic regions adjacent to the metallic core. Their optical properties are fundamentally controlled by shape: spherical particles exhibit a single plasmon band, while rod-shaped particles show split absorption bands corresponding to light absorption along short and long axes, with elongation causing red shifts toward the water window (750-1200 nm) for biological imaging. Three primary attachment methods enable biomolecular conjugation: electrostatic binding, EDC-mediated amide bond formation, and click chemistry (azide-alkyne cycloaddition). Each method affects protein activity differently, with click chemistry preserving higher enzymatic activity through steric shielding. Surface charge dramatically influences cellular interactions, with identical zeta potentials achieved through different ligands producing vastly different uptake efficiencies.

Surface modification creates reactive sites for subsequent chemistry through complementary approaches. Esterification couples carboxylic acids to surface hydroxyls using DCC/DMAP catalysts, requiring 24-hour reaction times. Silanization attaches functional groups via Si-O-Si bonds, completing in 3 hours using silane precursors. Amine coupling uses NHS esters or EDC to attach molecules to amine-functionalized surfaces. Gold surfaces enable direct functionalization without activation using bifunctional molecules (thiol + functional group). These techniques generate diverse surface chemistries including thiols, vitamins, amino acids, and click chemistry precursors. The choice between methods depends on substrate type, desired functional group, and application requirements, enabling customization from biosensors to catalytic surfaces.

Lateral flow immunoassays use porous strips with immobilized antibodies to detect antigens. Sample migration through the strip captures antigens at test lines, producing visible colored lines. Used in pregnancy tests (detecting hCG) and rapid COVID-19 antigen tests, these assays are simple, quick, and require no laboratory equipment. However, they can produce false positives in certain conditions—testicular cancer can produce antigens cross-reacting with pregnancy test antibodies. This case illustrates the importance of clinical context in interpreting diagnostic results and demonstrates how diagnostic tests can lead to important medical discoveries.

Photothermal therapy uses conducting polymer nanoparticles absorbing 700-900 nm light to generate heat for tumor destruction. Particles must be <200 nm and biocompatible. Studies with breast cancer cells (MDA-MB-231) show complete cell death at 10-50 μg/mL within 5 minutes of irradiation. Lower concentrations (0.5-1 μg/mL) require ~10 minutes. Concentrations below 55 μg/mL maintain cytocompatibility with normal cells (80% survival). This represents minimally invasive, site-specific treatment potentially superior to conventional therapies.

Lateral flow testing is a rapid diagnostic technique that has evolved from simple pregnancy tests to sophisticated multiplexed devices capable of testing multiple biomarkers simultaneously. Originally known primarily for pregnancy tests, the technology gained widespread awareness during the pandemic. The technique can be applied across infectious diseases, stroke monitoring, fertility testing, and many other medical areas. A key application is stroke differentiation, particularly large vessel occlusion, where tests can be performed in ambulances to inform hospitals before patient arrival. The technology has progressed from binary yes/no results to quantifiable measurements using smartphone apps or readers, working with multiple sample types including saliva, urine, and blood.

Rapid lateral flow immunoassays provide point-of-care serological testing for SARS-CoV-2 antibodies. A small blood sample is applied to a test strip containing capture antibodies for IgM and IgG. Buffer solution causes molecules to migrate through a nitrocellulose membrane. Control line detection confirms test validity. If IgM line appears, it indicates active infection. If only IgG line appears, it indicates past infection and potential immunity. This rapid testing enables quick triage decisions and epidemiological surveillance without requiring laboratory facilities.

Lateral flow immunoassays are primary binding tests providing rapid results for point-of-care diagnostics. These tests use colloidal gold particles as detection markers. Sample flows through a test strip containing capture antibodies and detection antibodies conjugated with gold particles. When target antigen is present, it forms a visible colored line at the test region. Applications include pregnancy tests (detecting hCG), rapid infectious disease tests, and veterinary diagnostics. While not all primary binding tests are rapid, all rapid tests are primary binding tests.

This section covers the fundamental principles of surface plasmon resonance in gold nanoparticles. Key topics include: (1) Gold nanoparticles can be synthesized into various shapes (nanospheres, nanorods, nanocubes, nanocages, nanostars) with distinct optical properties; (2) Nanoparticle size determines color through quantum size effects, with larger particles exhibiting red-shifted absorption peaks; (3) Surface plasmon resonance creates characteristic electric field distributions with strongest enhancement at specific surface locations; (4) Nanorods exhibit two distinct plasmon modes (longitudinal and transverse) producing separate absorption peaks, with peak positions dependent on aspect ratio; (5) COMSOL Multiphysics enables accurate simulation of nanoparticle absorption spectra, with experimental validation showing excellent agreement for particles around 32.9 nm average size.

This lecture explains how surface chemistry enables precise functionalization of plasmonic nanoparticles (such as gold nanoparticles and nanorods) for biosensing applications. The presentation covers nanoparticle synthesis methods including the classic Turkevich method for spherical gold nanoparticles and seed-mediated growth for anisotropic nanorods, which allow tuning of localized surface plasmon resonance (LSPR) wavelengths from 400-1000 nm. Key strategies for coupling biomolecules like antibodies to nanoparticles include electrostatic binding, silane chemistry, and protein A/G-mediated approaches, with optimal results achieved through controlled silane functionalization yielding approximately 4.4 antibodies per 15 nm nanoparticle. Two main biosensing approaches are demonstrated: colorimetric detection based on nanoparticle aggregation causing visible color changes, and surface plasmon resonance (SPR) detection monitoring LSPR wavelength shifts, achieving detection limits as low as 5 ng/mL for toxins in complex matrices like milk. For enhanced applications, silica coating techniques using sequential hydrolysis-condensation processes enable controlled porosity (parallel or perpendicular to nanoparticle axes) and improved stability, with hollow nanocontainer structures enabling drug delivery applications. The lecture emphasizes that proper surface functionalization combined with appropriate nanoparticle architecture allows achieving desired biosensing performance while maintaining colloidal stability and long-term functionality.

Gold nanorods are synthesized via seed-mediated growth: rapid reduction of gold chloride by sodium borohydride creates small crystal seeds, which then grow in the presence of CTAB surfactant and weak reducing agents. CTAB forms a charged bilayer stabilizing particles in water. Seed crystallinity critically affects outcomes—CTAB-reduced seeds are single-crystalline while citrate-reduced seeds are pentatwinned, producing different nanorod morphologies. Advanced characterization using electron microscopy, electron energy loss spectroscopy, and computational modeling reveals that lateral facets correspond to higher-index crystal planes rather than assumed (100)/(110) planes. Understanding these mechanisms enables precise control over nanoparticle morphology and properties.

Localized surface plasmon resonance utilizes metal nanoparticles where collective electron oscillations create strong electromagnetic fields at the particle surface. The optical properties depend on nanoparticle size, shape, and composition. Nanoparticles are fabricated using nanosphere lithography: polymeric nanospheres are deposited on substrates, silver is deposited around them, and sonication removes the spheres, leaving triangular nanoparticle structures. When resonant molecules bind to nanoparticles, coupling between molecular electronic transitions and plasmon oscillations produces dramatically enhanced wavelength shifts (~60 nm for resonant molecules versus ~20 nm for non-resonant molecules). Maximum enhancement occurs when molecular resonance is slightly red-shifted relative to nanoparticle plasmon resonance.

Localized surface plasmon resonance (LSPR) tunability is governed by multiple interconnected factors. Material selection fundamentally determines resonance position: silver exhibits blue-shifted LSPR due to low optical losses, while gold shows red-shifted LSPR. The surrounding medium's dielectric constant affects resonance through the polarizability formula, where ε_metal + 2×ε_dielectric appears in the denominator. Increasing refractive index from air to water causes red-shifting. Geometric engineering provides additional control: increasing nanorod aspect ratio shifts resonance to longer wavelengths, core-shell structures allow systematic tuning by varying shell thickness, and nanocage geometries enable compositional control. When plasmonic nanostructures interact, they form collective hybrid modes. The symmetric (bonding) mode has lower energy and exhibits a net dipole moment, making it strongly coupled to far-field radiation. The anti-symmetric (anti-bonding) mode has higher energy and reduced net dipole moment, leading to poor coupling efficiency. This symmetry-based classification provides predictive power for understanding complex plasmonic behavior.