Citrate-Coated Gold Nanoparticles, Spherical

Representation of a gold nanoparticle with citrate surface coating

Citrate-capped gold nanoparticles, 5 to 200 nm

Citrate-capped gold nanoparticles (AuNPs) are spherical gold particles carrying a weakly bound, negatively charged citrate layer formed during synthesis. Citrate-capped, citrate-coated and citrate-stabilized all describe this same surface. NanoBrand offers them in 16 sizes from 5 to 200 nm, supplied in Milli-Q water with no reactants, buffers or added stabilizers, and characterized lot by lot.

They are a common starting material for nanoparticle-based work in diagnostics, biosensing, imaging, drug delivery and cancer therapy research. The citrate surface holds the particles stable in dispersion. Particle size sets the optical response, the particle count per volume and how the particles move through a membrane in assays.

Which size for your application

Smaller spherical AuNPs may suit research applications where fast, uniform migration, efficient surface functionalisation, and reduced steric crowding are important. Larger particles provide stronger optical scattering and a higher visible signal at the same particle count, which can be useful in imaging, spectroscopy, colorimetric detection, and other optical applications.

For lateral flow assays and other IVD uses, particle size should be confirmed by testing the complete conjugate and membrane system, and not the bare particle alone. The optimal AuNP size depends on the intended application, surface chemistry, formulation, optical requirements, and final assay performance.

The 16 sizes below are the general-purpose citrate line. For particles prepared for passive adsorption or covalent EDC/NHS coupling in lateral flow and biosensors, see IVD-READY Gold Nanoparticles. For SERS, plasmonic sensing and photothermal research, see Gold Nanostars. Application notes for diagnostic and therapeutic work are on Biocompatible Nano-Gold.

Available Citrate-coated Spherical Gold Nanoparticle Products:

Shematic representation of gold nanoparticles monodispersity and lot-to-lot consistency, by matching nanoparticles diameters and UV-vis spectra

Citrate surface and dispersion medium

Citrate-stabilized gold nanoparticles (AuNPs) provide a practical starting surface for protein, peptide, and oligonucleotide conjugation. Proteins and biomolecules can attach through passive adsorption, while thiol-containing ligands form strong Au–S bonds for more controlled surface functionalization. The citrate layer is negatively charged and supports colloidal stability. It can be partly displaced or over-coated during ligand exchange, allowing passive adsorption and covalent AuNPs conjugation strategies to be developed from the same particle surface.

The dispersion medium can affect the nanoparticle surface as much as the capping agent. Surfactant or buffer residues may adsorb to gold nanoparticles and compete with ligands during conjugation. NanoBrand citrate gold nanoparticles are supplied without added surfactants or stabilizing agents, presenting a citrate-capped surface for protein, peptide, and oligonucleotide functionalization.

Learn more about True Citrate Surface and Why Stabilizing Agents should be Avoided.

Monodispersity and lot-to-lot consistency

Gold nanoparticles from 7 to 200 nm are supplied with a coefficient of variation (CoV) of 5% or less and a polydispersity index (PdI) of 0.1 or less. The 5 nm product has a CoV of 10% or less. These specifications are measured for each lot supplied. A narrow particle-size distribution supports consistent optical properties, helping experimental results remain comparable between orders. 

Each NanoBrand lot is supplied with a Certificate of Analysis (CoA) and Safety Data Sheet (SDS). The CoA includes lot-specific TEM images, size data and distribution, UV-vis spectrum, hydrodynamic diameter, polydispersity index (PdI), and Zeta potential, supporting traceable nanoparticle characterization. 

View a typical CoA for 20 nm Citrate-coated Gold Nanoparticles, compare Three Independent NanoBrand Lots, or request characterization for your own samples. 

Explore peer-reviewed scientific articles using our gold nanoparticles to see how they have been applied in diagnostics, imaging and therapeutic research.

picture of three bottles containing NanoBrand gold nanoparticles of different diameters

Configurations, pricing and ordering

Every size is listed at optical density 1, 10 and 20, in volumes from 10 ml to 1000 ml. Pricing depends on particle size, optical density and volume. The price for each configuration is shown on the product page and updates in the cart.

  • Universities: 10% discount applicable to the purchase order
  • Returning customers: 5% discount added to the cart automatically
  • Products are manufactured in Canada and ship in 1–2 weeks for small orders and 2–3 weeks for large orders 
  • Custom diameters and surface chemistries: please send us the diameter, surface coating, concentration as OD or particles per ml, and order volume
  • Bulk quantities: lead times are confirmed following review of the requested specifications, concentration, batch volume and characterization requirements

Frequently Asked Questions

What is the difference between the citrate line and IVD-Ready gold nanoparticles?

Both NanoBrand product lines provide high-quality gold nanoparticles with the same level of core characterization, but they are designed for different development and application needs.

IVD-READY gold nanoparticles are designed for conjugation and diagnostic assay development, including lateral flow assays, rapid tests, and biosensor platforms. They are available in 20, 40, 80, and 100 nm sizes at OD 20 or OD 50, with either a citrate surface for passive adsorption or a PEG5000-carboxylate surface for covalent EDC/NHS coupling to biomolecules with primary amines. Each lot is supplied with a lot-specific Certificate of Analysis (CoA) that includes defined validation ranges for key characterization variables, helping simplify method validation, lot qualification, and documentation from feasibility through verification, validation, and scale-up. See an example CoA for 40 nm IVD-Ready citrate-coated gold nanoparticles.

Citrate-coated gold nanoparticles are NanoBrand’s general-purpose AuNPs for research, bioconjugation, early assay development, passive adsorption, thiol–gold binding, and applications requiring further surface modification. They are available in 16 sizes from 5 to 200 nm, offering broad flexibility when particle-size selection is important. Each lot is supplied with a lot-specific CoA containing the relevant technical characterization data. See an example CoA for 40 nm citrate-coated gold nanoparticles.

What does optical density mean, and how does it relate to particles per ml?

Nanoparticle concentration is often expressed as optical density (OD), which measures how strongly a sample absorbs light at a specified wavelength. For gold nanoparticles of the same size and optical properties, OD is directly proportional to the number of particles per millilitre.

For example, an OD 10 dispersion of 20 nm gold nanoparticles contains approximately ten times more particles per mL than an OD 1 dispersion of the same 20 nm particles.

For each NanoBrand lot, the Certificate of Analysis provides the measured particle concentration (particles/mL), molar concentration, and gold mass concentration (e.g., mg/L).

Do you ship gold nanoparticles internationally?

Yes. NanoBrand manufactures in Canada and supplies customers worldwide. Prices display in US dollars for most countries and in the local currency for a number of European markets, set through the country selector on the site. Shipping costs are calculated at checkout once the destination is selected.

How should citrate gold nanoparticles be stored, and how long do they last?

Citrate gold nanoparticles arrive ready to use in liquid form. They do not require refrigerated transportation. Store them between 4 and 25 °C away from light and do not freeze, since a freeze-thaw cycle destabilizes the colloid. Kept properly, they can be used for at least one year. Beyond that, measurement of UV-vis spectra is recommended to confirm products stability. Our tutorial on evaluating gold nanoparticle stability with UV-vis spectroscopy shows how.

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