Increasing Lateral Flow Assay Sensitivity

Lateral flow assays offer speed, simplicity, and portability, but achieving a sufficiently low limit of detection can be challenging. A test may produce a strong signal at higher analyte concentrations yet fail to generate a clearly distinguishable test line near the clinically or analytically relevant cutoff.

Increasing lateral flow assay sensitivity often requires optimizing the assay as a whole rather than simply increasing antibody or particle loading. The detectable signal depends on target capture, nanoparticle optical properties, conjugate activity, flow behavior, non-specific background, and the amount of analyte that reaches the test line.

Several of these limitations may be addressed by selecting the right reporter nanoparticle. By tailoring the particle size, structure, or surface chemistry researchers can increase the signal generated by each binding event, improve conjugate performance, or concentrate low-abundance targets before detection.

Why do Lateral Flow Assays Struggle at Low Analyte Concentrations?

In a typical sandwich lateral flow assay, the target binds to a nanoparticle conjugate as it moves through the strip. The resulting complex is captured at the test line, where accumulated nanoparticles generate a visible or instrument-readable signal.

At low target concentrations, assay performance may be limited by:

• Insufficient signal from each captured particle
• Loss of antibody activity during conjugation or drying
• Poor conjugate release or membrane flow
• Non-specific binding that raises background
• Inefficient target capture
• Interference from the sample matrix

Because these factors interact, no single nanoparticle performs optimally across all lateral flow assays. Instead, developers should prioritize the assay’s primary performance limitation when optimizing nanoparticle selection.

Evaluating Gold Nanosphere Size

Gold nanospheres remain the most widely used nanoparticles in lateral flow assays because they provide a visible signal without requiring an external reader. However, particle size influences optical intensity, available surface area, colloidal stability, and membrane transport.

Smaller gold nanoparticles migrate easily, but each particle produces a relatively modest optical signal. Larger gold nanospheres generate greater signal per particle, which can improve test-line visibility when fewer particles are captured.

Increasing lateral flow assay sensitivity through particle-size optimization requires balancing stronger optical output with reliable release and run time. For many assays, 40 nm gold nanospheres provide a useful starting point because they balance optical intensity, conjugation capacity, stability, and flow. Assays requiring a stronger visual signal may benefit from evaluating 80 nm gold nanospheres.

Increasing Signal per Binding Event with Gold Nanoshells

When traditional gold nanospheres cannot generate sufficient contrast at the required cutoff, gold nanoshells offer a promising strategy to increase lateral flow assay sensitivity.^1,2

150 nm gold nanoshells are engineered to deliver a strong optical signal per particle in lateral flow assays. A silica core surrounded by a thin gold shell provides high extinction without the mass of a similarly sized solid gold particle. This low-density architecture helps the larger nanoshells flow efficiently along lateral flow strips, with migration behavior comparable to gold nanospheres.

Figure 1. Per particle extinction spectra of 150 nm gold nanoshells compared with 40 nm gold nanospheres. When normalized on a per particle basis, each gold nanoshell produces significantly greater optical signal than a 40 nm gold nanosphere.

This makes gold nanoshells especially useful when:

• The assay detects high analyte concentrations but not at the desired limit of detection
• Increasing conjugate concentration also increases background
• A reader-free format must be maintained
• A reduced amount of antibody or conjugate required per test is desired

Gold nanoshells may still require optimization of membrane pore structure, surfactant concentration, conjugate loading, and release conditions. However, when weak optical signal is the primary limitation, nanoshells can lower the limit of detection without fundamentally changing the assay format.

Simplifying Assay Development with Application-Ready Gold Conjugates

Ready-to-use conjugates can reduce the variability associated with developing and optimizing a conjugation process from scratch.

40 nm streptavidin gold and 150 nm streptavidin gold nanoshells conjugates provide a flexible way to attach biotinylated antibodies, proteins, or oligonucleotides. 40 nm Protein G gold conjugates bind the Fc region of IgG antibodies, orienting the antigen-binding regions outward for more efficient target capture.

Figure 2. Comparison of BioReady^TM 40 nm streptavidin-gold conjugate with other suppliers'. In a lateral flow format, only the nanoComposix conjugate produced a strong visible signal at 0.01 nM analyte concentration. At 1 nM, only one supplier’s conjugate produced signal, quantified to be less than 20% of the signal generated by the nanoComposix conjugate.

In nanoComposix testing, our ready-to-use streptavidin and Protein G gold conjugates demonstrate increased sensitivity and decreased background compared with alternative suppliers’ conjugates. By improving the balance between specific signal and background, these conjugates can make low analyte concentrations easier to distinguish from negative samples.

Ready-to-use conjugates can help developers:

• Reduce non-specific binding
• Improve signal-to-background performance
• Increase lateral flow assay sensitivity
• Screen recognition reagents more quickly
• Reduce conjugation-related variability
• Separate conjugation challenges from other assay-development limitations

For developers focused on increasing lateral flow assay sensitivity, ready-to-use conjugates provide a consistent platform for optimizing antibody pairing, membrane selection, running buffer, and target capture.

Concentrating Low-Abundance Targets with Magnetic Gold Nanoshells

In some assays, the optical reporter is not the main limitation, it’s the amount of target reaching the test line.

Carboxyl magnetic gold nanoshells can capture analytes from a larger sample volume before lateral flow analysis. The isolated target can then be concentrated into a smaller volume and applied to the strip. Magnetic separation may also help remove sample components that interfere with binding or flow.

Magnetic enrichment may be valuable when:

• Optical enhancement alone is insufficient
• The target is present at very low concentrations
• Larger sample volumes are available
• Matrix interference limits direct testing
• A short sample-preparation step is acceptable

Magnetic enrichment adds a processing step, but can address a limitation that stronger optical labels cannot solve: too little analyte entering the assay.

Matching the Nanoparticle Strategy to the Sensitivity Bottleneck

A low detection limit rarely depends on a single variable. Before selecting a new particle, determine which factor is most likely limiting performance.

References

1. Srinivasan, B.; Nanus, D. M.; Erickson, D.; Mehta, S. Highly Portable Quantitative Screening Test for Prostate-Specific Antigen at Point of Care . Curr. Res. Biotechnol. 2021, 3, 288–299. https://doi.org/10.1016/j.crbiot.2021.11.003.
2. Pedram, B.; Pasquetto, V.; Drame, P. M.; Ji, Y.; Gonzalez-Moa, M. J.; Baldwin, R. K.; Nutman, T. B.; Biamonte, M. A. A Novel Rapid Test for Detecting Antibody Responses to Loa Loa Infections . PLoS Negl. Trop. Dis. 2017, 11 (7), e0005741. https://doi.org/10.1371/journal.pntd.0005741.