Gold Nanoparticle Labels Custom Designed for Lateral Flow Assays

Lateral flow assays (also known as lateral flow immunoassays, lateral flow tests,
immunochromatographic assays, and rapid strip tests) are a user-friendly test format designed to detect the presence or absence of a target analyte within a sample. They can detect a wide variety of pathogens, drugs, hormones, metabolites, and other molecules from biological and chemical samples.

Lateral Flow Assays provide test results quickly, offer long-term stability over a wide range of climates, and are relatively inexpensive to make. These benefits make them ideal for home testing (such as pregnancy tests), rapid point of care testing, and field testing for various environmental and agricultural analytes. Currently, Lateral Flow Assays are used in a variety of applications in human and veterinary medicine, food and beverage manufacturing, pharmaceuticals, personal care product manufacturing, environmental remediation, and water utilities. They are also used extensively in testing for biowarfare agents and pathogens such as anthrax, smallpox, avian influenza, and other potential biological weapons.

How Lateral Flow Assays Work

Lateral flow tests, or lateral flow assays (LFAs) are rapid diagnostic assays that do not require any special machinery to run or provide a readout. They are simple devices that provide a visual readout and is the preferred test for low-cost and/or portable applications. Typically, lateral flow test strips are composed of a sample pad, conjugate pad, reaction membrane, and absorbent pad. A sample is placed on the sample pad at one end of the strip and then flows to the conjugate pad and mixes with the visual indicator. The solution is then moved to the reaction membrane (generally nitrocellulose) and interact with a test line and a control line. There are a number of different types of indicators, but typically gold nanoparticles are the indicator of choice because they provide excellent sensitivity.

Competition LFA - These are the most common LFA format and are well suited for small analytes with only one available binding site. For this class of LFA, a positive test will yield no color. A negative test will have a colored test line.

One type of competition assay will conjugate the affinity molecule on the reporter (gold nanoparticle in this case). The test strip will contain immobilized analyte. If the test analyte exists in the sample, then the reporters will not bind to the test line on the strip indicating a positive test. If no analyte exists in the test solution, then the reporter binds to the strip indicating a negative test.

Another type of competition assay will conjugate the analyte to the reporter. The test strip will contain immobilized affinity molecules. If the test analyte exists in the sample, then the analyte will bind to the immobilized affinity molecule on the test strip. This will inhibit the analyte conjugated to the reporter to bind to the test strip, indicating a positive test. If no analyte is present in the sample, then the reporters will bind to the test strip, indicating a negative.

Sandwich LFAs - Ideal for large analytes with multiple binding sites. For this class of LFA, a positive test will yield a colored test line. A negative test will have no color on the test line.

 These assays have the affinity molecule both conjugated to the reporter and immobilized on the test strip. If the analyte is present in the sample, then both the test strip and reporter will bind to it, giving a high contrast line indicative of a positive test. If no analyte is present in the sample, then the reporters do not accumulate on the test line on the strip, indicating a negative test.

Gold NanoSphere Labels 

Careful selection of label is critical to the performance of any lateral flow assay. Gold nanospheres are the most widely used lateral flow indicators because of their extraordinary chemical and optical properties. The chemically inert nature of gold enables gold nanospheres to maintain exceptional stability against degradation for extended periods of time.

Gold nanospheres also have a very high affinity for biomolecules, enabling quick and durable conjugation of antibodies, aptamers, and other targeting moieties commonly used for lateral flow tests. Furthermore, numerous techniques are available to functionalize gold nanospheres, which enable more for more advanced bioconjugation strategies to be performed to improve distribution, density, specificity, and composition of targeting biomolecule conjugates. Gold nanoparticles also exhibit a strong surface plasmon reference (SPR) making them excellent lateral flow test indicators.

The 'Gold' Standard for Lateral Flow Assays

Though gold nanospheres in the 5 - 150 nm size range can be used for lateral flow assays, a perfect balance must be struck between size, sensitivity and colloidal stability of the gold nanosphere labels. Generally, small gold nanospheres absorb at lower wavelengths (~510-520 nm), while larger gold nanospheres absorb at longer wavelengths (beyond 600 nm). Since larger nanospheres have a higher magnitude of absorption and available surface area for antibody conjugation, they can provide better assay sensitivity. However, when larger nanospheres are used, absorption in the longer wavelengths reduces the contrast on the test strip.

40 nm Gold NanoSpheres are the most popular choice for lateral flow assays. This is due to an optimal combination of high contrast color (absorption ~523 nm) which appears cherry red and surface area (minimize wasted targeting biomolecules) for effective and efficient analyte testing, 

30 nm and 60 nm Gold NanoSpheres also have specific advantages depending on the design and application of the lateral flow test.

30 nm Gold NanoSpheres (LSPR peak wavelength of 519 nm) have a darker red appearance that improves contrast against white lateral flow assay membranes compared to 40 nm spheres (LSPR peak wavelength of 523 nm). With increased contrast, due to the smaller size 30 nm Gold NanoSpheres require more conjugated antibodies to achieve an equivalent mass concentration. Therefore, 30 nm Gold NanoSpheres are an excellent option for applications with low antibody costs and plentiful target analyte. 

Lateral Flow Assay Performance

Stringent Quality Standards for Better Assay Performance

The quality of gold nanoparticles can have profound effects on the specificity, sensitivity and reproducibility of lateral flow assays.

NanoHybrids has specifically engineered Gold NanoSpheres to deliver the most reproducible, consistent, and high quality performance available. 

Monodispersity: Seed-mediated manufacturing produces a more monodisperse, spherical product with an absorbance peak of 523 nm and an 80% spectral bandwidth of less than 37 nm for our 40 nm Gold NanoSpheres and an absorbance peak of 533.5 nm and an 80% spectral bandwidth of less than 41 nm for our 60 nm Gold NanoSpheres.

Consistency: Industry-leading minimization of batch-to-batch variability (+/- 1.5 nm) and size distribution (CV < 10% for 40 nm and 30 nm Gold NanoSpheres and CV < 8% for 60 nm NanoSpheres)*.

Purity: Thorough purification ensures that the final produce is greater than 99.999% reactant free.

This dedication to quality translates to dependable and consistently superior assay results. It also ensures better control of gold nanoparticle surface area and consistent flow dynamics across membranes along with high sensitivity from even binding of antibody or multiplexing modifications.

* Coefficient of variation (CV) is the standard deviation divided by the average size of the nanoparticle population. Note the standard deviation is less than +/- 4 nm for 40 nm NanoSpheres and 4.8 nm for 60 nm NanoSpheres. 

Thorough Characterization

Every batch is completely characterized using TEM, DLS, and UV-Vis spectrophotometry and carefully tested for aggregation and residual chemicals. Batch-specific CoA sheets provide TEM images, TEM measured size, zeta potential, pH, concentration (Optical Density or OD, molarity, particles/ml and mg/ml), and optical spectra, so you know exactly what you are getting with each order.

Available in High Concentrations

By ordering highly concentrated colloidal gold nanoparticles the concentration process is avoided and nanoparticles can be directly coated antibodies, proteins or other moeities reducing both waste and labor. Concentrated gold nanoparticles can also help create denser, more uniform layers of gold nanoparticles on the membrane.

Scale-up capability

At NanoHybrids, we scale up without compromising particle monodispersity or quality. Batch sizes in liters are available upon request. Bulk nanoparticles retain a long shelf life (>1 year) which enables a consistent test manufacturing regime with volume cost savings.

Technical Assistance for Assay Development & Troubleshooting + Custom Conjugations

The scientists at NanoHybrids are available to answer any questions you might have related to choosing the right gold nanoparticles, buffers and antibodies for assay development. Custom nanoparticle modifucations are also available upon request for assay development and optimization. 

Depending on the assay design, NanoHybrids also offers custom conjugation to antibodies, proteins, affibodies, aptamers or other moeities.

 Contact NanoHybrids

Contact us to get additional information about our gold nanoparticles for Lateral Flow Assays.