40 nm Gold Nanospheres for Conjugation

Product Number


  • All BioReady products are specially formulated for lateral flow and biodiagnostics
  • Mean diameter: 40 ± 4 nm
  • Size distribution (CV) < 15%
  • Surfaces available:
    • Carbonate, provided at 4.5 ± 0.15 OD in aqueous carbonate
    • NHS, provided as a dried powder with reaction kit components included (small reaction kits reconstitute to 50 µL of 20 OD gold for each unit; large reaction kits reconstitute to 500 µL of 20 OD gold for each unit); check out our video tutorial on how to use these particles
    • Carboxyl (–COOH), provided at 20 OD in DI water
  • Looking for greater sensitivity?

Which Surface Should I Choose?

Surface Binding type Pros Cons
Carbonate Learn more + Passive adsorption
(aka physisorption)
  • Simple conjugation method (mix and wait)
  • Low cost
  • Less stable in various pH/buffer conditions than covalent bonding
  • Requires more antibody per particle

BioReady carbonate gold nanoparticles have a “naked” particle surface with only a weakly bound carbonate molecule stabilizing the particle. Similar to citrate-stabilized gold, the carbonate is readily displaced in the presence of proteins for passive adsorption (also referred to as physisorption). The mechanism of adsorption is based on Van der Waals interactions between the proteins (e.g. antibodies) and the surface of the particles. The resulting forces between the antibody and the nanoparticle are influenced by the coupling environment. BioReady carbonate gold is provided at an optical density (OD) of 4 at pH ~8.0–8.5, which is optimal for conjugation to many IgG antibodies. A pH titration can be performed to optimize the pH of conjugation.

NHS Learn more + Covalent
  • Simplest covalent conjugation procedure
  • Lyophilized powder format
  • Best for rapidly fabricating and testing conjugates
  • Must be used immediately after reconstitution with water
  • More expensive than Carboxylic Acid at larger scales

BioReady NHS gold can be covalently conjugated to primary amines (–NH2) of proteins in a simplified procedure. Covalent coupling of proteins (e.g. antibodies) to a gold nanoparticle surface yields robust and reliable gold particle conjugates. The BioReady NHS gold nanoparticles are surface functionalized with an active NHS ester to generate gold nanoparticle-antibody amide bonds, eliminating the need for the user to perform the intermediary EDC/Sulfo-NHS chemistry steps. The particles are supplied as a lyophilized powder that can be resuspended with a buffer to covalently bind to an added antibody. This coupling reaction is rapid, simple, robust and requires little optimization.

The following is included in every NHS reaction kit:

  • Antibody Purification Buffer (10 mM potassium phosphate, pH 7.4)
  • Reaction Buffer (5 mM potassium phosphate, 0.5% 20K MW PEG at pH 7.4)
  • Quencher (5% (w/v) hydroxylamine)
  • Conjugate Diluent (0.1× PBS, 0.5% BSA)

Antibody purification spin filters (Amicon® Ultra 0.5 mL Filters 10 kDa) may also be purchased with your nanoparticles.

Carboxyl Learn more + Covalent
  • Maximizes sensitivity
  • More cost effective at large production scales
  • Requires additional EDC/Sulfo-NHS activation step

BioReady carboxyl (–COOH) gold is an effective and economical nanoparticle for covalent conjugations to proteins through carbodiimide crosslinker chemistry. Covalent coupling of proteins (e.g. antibodies) to a gold nanoparticle surface yields robust and stable gold particle conjugates. The nanoparticles are surface functionalized with a tightly bound monolayer that contains terminal carboxylic acid functional groups which can be activated through EDC/Sulfo-NHS chemistry to generate gold nanoparticle-antibody amide bonds. The EDC/Sulfo-NHS chemistry requires an extra step to activate the carboxyl group but can be performed at scale at reasonable cost, making carboxyl gold the most cost-effective method of covalently binding antibodies to gold.

Certificate of Analysis Examples

Please note that these are representative Certificates of Analyses (CoAs) provided as examples for this product. We provide a unique batch-specific CoA with each product during shipment; only the CoA that arrives with your product should be referred to for actual characterization and measurement data. If you would like an electronic copy of the CoA for the product you received or the material(s) we currently have in stock, please contact us.

Product Line Surface Example Certificate of Analysis Product # Price
BioReady Carbonate, 4.5 OD Download ↓ AURR40-25M $75+
BioReady Carbonate, 4.5 OD Download ↓ AURR40-100M $275+
BioReady Carbonate, 4.5 OD Download ↓ AURR40-1000M $2,395+
BioReady NHS, Dried Download ↓ AUNR40-3SR $95+
BioReady NHS, Dried Download ↓ AUNR40-3SRF $120+
BioReady NHS, Dried Download ↓ AUNR40-10SR $245+
BioReady NHS, Dried Download ↓ AUNR40-10SRF $315+
BioReady NHS, Dried Download ↓ AUNR40-1LR $225+
BioReady NHS, Dried Download ↓ AUNR40-1LRF $235+
BioReady Carboxyl, 20 OD Download ↓ AUXR40-5M $125+
BioReady Carboxyl, 20 OD Download ↓ AUXR40-30M $535+
BioReady Carboxyl, 20 OD Download ↓ AUXR40-100M $1,495+

Need help reading our Certificates of Analysis? +

Gold Nanoparticle Applications

Diagnostic Applications

Gold nanopaticles are readily conjugated to antibodies and other proteins due to the affinity of sulfhydyl (-SH) groups for the gold surface, and gold-biomolecule conjugates have been widely incorporated into diagnostic applications, where their bright red color is used in home and point-of-care tests such as lateral flow assays.  

Biomedical Applications

Gold nanomaterials can be conjugated to biomolecules to specifically target cancer cells, and used for photothermal cancer therapy, where their tunable optical properties cause them to convert laser light into heat and selectively kill cancerous cells.   


Gold nanoparticles have unique optical properties because they support surface plasmons. At specific wavelengths of light the surface plasmons are driven into resonance and strongly absorb or scatter incident light. This effect is so strong that it allows for individual nanoparticles as small as 30 nm in diameter to be imaged using a conventional dark field microscope.  This strong coupling of metal nanostructures with light is the basis for the new field of plasmonics.  Applications of plasmonic gold nanoparticles include biomedical labels, sensors, and detectors. The gold plasmon resonance is also the basis for enhanced spectroscopy techniques such as Surface Enhanced Raman Spectroscopy (SERS) and Surface Enhanced Fluoressence Spectroscopy which can be used to detect analytes with ultrahigh sensitivity. 

Ultra Uniform Gold Nanospheres

Our standard Au nanospheres have a variety of useful surface chemistries and narrow size distributions (CV < 15%), appropriate for a variety of applications ranging from lateral flow assays to optical coatings. Some applications, however, such as multiplexed dark field labeling or standards for nanoparticle size and shape, require even tighter tolerances on shape and size dispersity. For these applications, nanoComposix has successfully manufactured ultra-uniform, monodisperse gold nanospheres with sizes tunable from 10 nm up to 200 nm. As shown below, these gold nanoparticles have a nearly perfect spherical shape, smooth surfaces, and impressively narrow size distributions (CV < 6%, and for many sizes CV < 4%).

Contact us to learn more

Optical and Scattering Properties

Due to their high uniformity in size and shape, these Au nanospheres scatter a single color of light under dark field microscope imaging (shown below). The high purity of their light scattering signatures makes these nanospheres perfect scattering labels for imaging and building blocks for plasmonic nanostructures and devices.

Surface Chemistry

The gold nanospheres are stabilized with citrate in an aqueous solution. With their stable, easily displaceable surface chemistry, the gold nanospheres can be readily functionalized with a wide variety of molecules, including polymers and small molecules with desired functional groups, inorganic coatings such as silica, and biomolecules including DNA and antibodies for your application.

Contact us to learn more

Need Greater Sensitivity?

Gold nanoshells typically provide 2–20× increases in lateral flow diagnostic assay sensitivity versus 40 nm gold nanoshells. The gold nanoshells consist of a 120 nm silica core that is coated with a 15 nm gold shell. The nanoshells are less dense than solid 150 nm gold spheres and are highly uniform resulting in excellent flow characteristics. Since the particles have the same gold surface as 40 nm gold nanospheres, only minor changes to conjugation protocols are required in order to generate ultrasensitive assays. The price per OD-mL of the nanoshells is only slightly higher than our 40 nm gold nanoparticles.

For more information about increasing your assay sensitivity with gold nanoshells, see our BioReady Gold nanoshell page.

Nanoparticles for Lateral Flow

nanoComposix has extensive expertise in the synthesis, characterization and surface modification of nanoparticles. We have been making highly engineered inorganic particles for more than ten years and have developed particles specifically engineered for both their optical properties and the conjugation to affinity ligands such as antibodies.

The methods and techniques used to conjugate antibodies to the surface of nanoparticles are critical for optimizing the performance of lateral flow assay tests. For gold nanoparticles, antibodies can either be physisorbed to the surface, referred to as passive adsorption, or they can be covalently coupled. In both passive and covalent coupling reactions, the purity, affinity, and cross-reactivity of an antibody or other ligand is important for developing sensitive and specific tests. Typically we purify all antibodies before use in a coupling reaction.

It is important to note that the guidance provided here is specific to conjugation procedures for binding antibodies to gold nanoparticles. While antibodies are the most common affinity ligand used in lateral flow tests other molecules can also be attached to nanoparticles, such as small peptides and other proteins (BSA, streptavidin, etc.).

Passive Adsorption

This is the original method for attachment of proteins to lateral flow nanoparticle probes and is still widely used. The mechanism of adsorption is based on Van der Waals and other attractions between the targeting ligand and the surface of the particles. The resulting forces between the antibody and the nanoparticle probe are influenced both by the nanoparticle surface and by the coupling environment. In the case of less hydrophobic antibodies or a more hydrophilic surface (i.e. –COOH modified), attachment by both ionic interactions and hydrophobic interactions can occur. Small changes in pH can alter the association dynamics and affect the efficiency of conjugation. Typically, a pH titration and an antibody loading sweep are performed to identify conditions where antibody absorption is optimal. It is recommended that the pH of the adsorption buffer is slightly above the isoelectric point of the protein (which varies from antibody to antibody). When working with antibodies, the Fc portion is generally more hydrophobic and more likely to be adsorbed as compared to the Fab portion offering some control over binding orientation. A large excess of antibody with respect to nanoparticle surface area is typically used in order to ensure dense surface binding and high salt stability post conjugation. There are two main drawbacks to the passive adsorption. Firstly, every antibody requires slightly different conditions and secondly there is a certain amount of lability of physisorbed antibodies allowing for some antibodies to be released from the nanoparticle surface which can lead to a decrease in sensitivity and variable results.

Covalent Coupling

Increasingly, LFA developers are covalently binding antibodies to the surface of nanoparticle lateral flow probes. Covalent attachment is more stable with less antibody desorption and requires fewer antibodies during conjugation. Covalent attachment can be accomplished with several different chemistries. For our BioReady products that are optimized for lateral flow, we typically utilize amide bonds to connect a carboxylic acid functionalized nanoparticle to free amines on the antibody. This covalent bond is achieved through an EDC/Sulfo-NHS intermediary (see Figure 5) generated from a carboxylic acid surface particle. For antibodies, lysine residues are the primary target sites for EDC/NHS conjugation. A typical IgG antibody will have 80–100 lysine residues of which 30–40 will be accessible for EDC/NHS binding. Proteins such as bovine serum albumin have similar numbers of surface accessible lysine groups. NanoComposix sells BioReady nanoparticles with both carboxylic acid surfaces and also an NHS activated surface to allow for simplified conjugation eliminating the need for the user to perform the intermediary EDC/NHS chemistry steps. We also have a video tutorial that outlines how to conjugate our NHS activated particles to your antibody. In addition to its use in lateral flow, the same particle surface chemistry can be used to bind many other amine containing targeting ligands to the particle surface.