Gold Nanoparticles: Synthesis, Unique Properties, and Biomedical Applications

Why Gold at the Nanoscale Is Nothing Like Bulk Gold

Bulk gold is chemically inert, yellow in colour, and a moderate electrical conductor. Gold nanoparticles (AuNPs) at 10–100 nm behave almost like a different material. They absorb and scatter visible light intensely (appearing red to purple in solution), they can catalyse reactions that bulk gold cannot, and their surface chemistry is exquisitely tuneable through thiol-based self-assembled monolayers. These emergent properties arise from two quantum phenomena: surface plasmon resonance (SPR) and the high fraction of surface atoms relative to the total.

Gold's long history of safe biological use — gold salts have been used medicinally for centuries — combined with these nanoscale properties has made AuNPs one of the most studied and commercially deployed nanomaterials in biomedicine.

Surface Plasmon Resonance: The Source of the Colour

When light strikes a gold nanoparticle, the conduction electrons collectively oscillate in resonance with the electromagnetic field — a phenomenon called localised surface plasmon resonance (LSPR). This resonance produces an intense absorption band whose peak wavelength depends on particle size, shape, and the surrounding medium:

• Spherical AuNPs 10–20 nm — SPR peak ~515–520 nm (red solution)
• Spherical AuNPs 80–100 nm — SPR peak ~570–580 nm (deeper red/purple)
• Gold nanorods — two SPR bands: transverse (~520 nm) and longitudinal (tunable 600–900 nm by controlling aspect ratio)
• Gold nanostars — multiple sharp tips create SPR peaks extending into the near-infrared; "hot spots" of extreme field enhancement
• Gold nanocages/nanoshells — hollow or silica-core structures with SPR tunable across the near-infrared window (700–900 nm) used for photothermal therapy

Synthesis Methods

The dominant wet-chemistry synthesis routes for spherical AuNPs:

• Turkevich method (citrate reduction) — HAuCl₄ reduced by trisodium citrate in boiling water. Simple, reproducible, produces 10–20 nm citrate-capped particles. Citrate acts as both reductant and stabiliser. The standard method for research-grade AuNPs since 1951.
• Brust-Schiffrin method (two-phase) — gold transferred to organic phase and reduced in presence of alkanethiols. Produces very small (1–5 nm), extremely stable, thiol-capped particles. Preferred for organic-soluble and functionalised applications.
• Seed-mediated growth — small seed particles (2–4 nm) are grown in a growth solution to achieve controlled sizes from 10 to 150 nm with narrow size distributions. The primary route for larger monodisperse particles.
• Gold nanorod synthesis (CTAB-assisted) — cetyltrimethylammonium bromide (CTAB) directs anisotropic growth in a seed-mediated process; aspect ratio controlled by silver ion concentration and reduction conditions.

Surface Functionalisation

AuNPs are functionalised via gold-thiol chemistry: thiol groups (R-SH) form strong, stable Au-S bonds that self-assemble into dense monolayers on the particle surface. This chemistry is mild, versatile, and well-characterised. Common functionalisations:

• PEG (polyethylene glycol) coating — improves colloidal stability in physiological media, reduces non-specific protein adsorption ("stealth" coating), extends circulation time in vivo. Essential for most biomedical applications.
• Antibody conjugation — anti-IgG secondary antibodies or primary antibodies coupled to AuNPs for immunoassays and cell targeting
• Streptavidin coating — high-affinity binding of biotinylated molecules; universal bioconjugation handle
• DNA / oligonucleotide conjugation — DNA-AuNP conjugates for gene regulation (spherical nucleic acids), diagnostics, and programmable self-assembly
• Peptide and small molecule conjugation — targeting ligands for receptor-mediated cellular uptake

Lateral Flow Assays and Point-of-Care Diagnostics

The red-coloured visible band you see on a pregnancy test or COVID-19 rapid test is formed by gold nanoparticles accumulating at the test line — aggregated AuNPs conjugated to antibodies that capture the target analyte. This application alone accounts for a substantial fraction of commercial AuNP consumption. Key advantages for lateral flow:

• Intense visible colour at low concentrations (~0.01 nM detection without amplification)
• Stable dry storage in conjugate pads for years
• No equipment needed for visual readout; smartphone readers enable quantitation
• Well-established conjugation protocols with anti-IgG or streptavidin

Photothermal Cancer Therapy

Gold nanorods, nanostars, nanocages, and nanoshells absorb near-infrared light (700–900 nm, the "biological window" where tissue is relatively transparent) and efficiently convert it to heat. When AuNPs are concentrated in tumour tissue — either by passive accumulation via the enhanced permeability and retention (EPR) effect or by active targeting via surface ligands — NIR laser irradiation selectively heats and ablates the tumour. Aurolase (gold nanoshell therapy by Nanospectra Biosciences) has completed multiple clinical trials for prostate cancer and recurrent head and neck cancers.

Drug Delivery

AuNPs serve as carriers for small molecule drugs, nucleic acids, and proteins. The high surface area, modular surface chemistry, and cellular uptake via endocytosis make them attractive vehicles. Drug release can be triggered by:

• NIR laser irradiation (photothermal cleavage of thiol-drug linkers)
• pH change in the endosome (acid-labile linkers)
• Glutathione concentration (intracellular disulfide cleavage)
• Ultrasound or external field application

SERS-Based Diagnostics

Surface-enhanced Raman scattering (SERS) substrates — typically aggregated AuNPs or nanostructured gold surfaces — amplify Raman signals by factors of 10⁶–10¹⁰ in the "hot spots" between particles. This enables single-molecule detection and multiplexed biomarker analysis from blood, urine, or tissue samples. SERS nanotags (AuNPs labelled with Raman reporter molecules) are in development for multiplexed tumour imaging and liquid biopsy assays.

Specifications to Verify When Sourcing AuNPs

1. Particle diameter (TEM) — mean and standard deviation
2. SPR peak wavelength (UV-Vis) — confirms size and confirms monodispersity
3. Concentration (OD at SPR peak, or particles/mL)
4. Surface coating (citrate, PEG, streptavidin, etc.) and ligand density
5. Hydrodynamic diameter (DLS) — should be consistent with TEM + coating thickness
6. Zeta potential — indicator of colloidal stability (|ζ| > 30 mV preferred)
7. Endotoxin level (EU/mL) — critical for any cell culture or in vivo use
8. pH and storage buffer