What is a Gator XT biosensor?

XT optics is what sets gator apart from the competition

Author: Benjamin Osborn, PhD; Anuja Patil, MS; Indrani Chakraborty, PhD

What is a BLI biosensor?

A biosensor is a detection instrument that uses a biological molecule to detect a specific target. These detector biomolecules (called “capture molecules” in this article) can include bacterial proteins like Protein A, antibodies/VHH and aptamers, each of which target molecules selectively. This allows biosensors to discriminate between samples that do contain the target molecule from samples that do not.

Biolayer Interferometry (BLI) is how selective binding is transduced into a meaningful signal. The inventor of BLI technology, Gator Bio’s CEO Hong Tan, discovered that when white light travels to the end of an optical fiber, a small layer of biomolecules (a biolayer) there reflects the light back into the fiber. A spectrometer can then observe that reflection.

A reflection isn’t a signal, but with the addition of a reference layer at the distal end of the biosensor, the two reflections (biolayer and reference) interfere with one another, changing the reflected light. The starting interference looks something like what’s pictured in Figure 1 in light grey. We already see some wavelengths amplified because the distance to and from the biolayer aligns with those wavelengths, creating constructive interference. Similarly, destructive interference occurs where that round-trip distance comes out to a half wavelength.

Figure 1: Exaggerated schematic representation of how antibody binding to a biosensor alters the interference spectrum in the visible range. Starting from a reference interference spectrum (grey), further biomolecule binding moves the peaks in the interference spectrum toward the infrared. This shift is plotted on a sensorgram as a function of time. Examples of sensorgrams are included in Figures 3 and 5.

As ligands bind to the surface, the peaks in the interference spectrum slide toward longer wavelengths (reds). Binding increases the optical thickness of the biolayer, which is its physical thickness multiplied by its refractive index change, so the reflected light completes a slightly longer round trip. The instrument reports that change in optical thickness in nanometers as the “nm shift.” Because optical thickness scales with the amount of material on the surface, the nm shift is a direct measure of how much biomolecule has accumulated.

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How are Gator XT biosensors different?

XT biosensor optics is what sets Gator BLI apart from existing BLI technologies with dramatically improved sensitivity and performance.

When Dr. Tan revisited BLI technology and founded Gator Bio, he wanted to address a problem he had noticed early on in its development. He was getting far less signal than he had expected given the sizes of molecules that were binding to the surface. This is because traditional biosensor design captures less light from the biolayer than from the reference layer. While reduced sensitivity wasn’t a problem for a first-of-its-kind instrument that mostly focused on large molecule quantitation and binding kinetics, first-gen BLI biosensors clearly still had room for improvements.

So when Dr. Tan returned to the problem and founded Gator Bio, the Gator team revisited this discrepancy. Could our engineers capture more light from the biolayer? Could we improve performance of a BLI biosensor solely by improving the optics? Over the following years, the Gator team completely redesigned the surface of the biosensor, culminating in a proprietary process that improved the reflection from the biolayer back into the instrument. Without getting too technical, these improvements allow more of the light from the biolayer reflection into the optical fiber to reach the spectrometer, increasing the signal intensity (see Figure 2).

Figure 2: Schematic representation of a traditional biosensor (top) vs a Gator XT biosensor (bottom). Note how the total reflection from the protein layer (orange) has a much greater amplitude from XT biosensors which capture more of the reflected light, resulting in a much larger phase shift (nm shift). For this reason, XT biosensors give more signal per molecule

Now that the reflection from the biolayer is more intense, the resulting nm shift in the interference spectrum is approximately 3x greater relative to a traditional BLI biosensor. Figure 3 shows a head-to-head comparison of a traditional streptavidin BLI biosensor against a Gator SA XT biosensor featuring these new optical improvements. The difference in sensitivity and performance is clear just from this single experiment – a 3-4 nm loading shift was observed with a traditional BLI optical design, versus a 12 nm loading shift using Gator XT optics. This improvement was observable on the base model Gator Prime instrument (launched several years before any biosensors with XT optics were available), without requiring any redesign of the instrumentation or software.

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Figure 3: Traditional BLI streptavidin sensors vs Gator Streptavidin XT. While the response is much greater on a Streptavidin XT biosensor, the antigen binding kon remains unchanged.

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Does increased sensitivity also mean increased noise?

Noise from biosensors with XT optics is still well beneath the instrument spec of 4 picometers of RMS noise, and the improvement to sensitivity dwarfs that small increase. As an example, across a panel of twenty-plus His-tagged proteins, we tested our anti-His XT (HIS XT) biosensor against both First-Gen BLI anti-his and our own traditional Gator Anti-His biosensor (See Figure 4 – full dataset here). The jump from First-Gen BLI to HIS XT is over 11x total signal on average. Moreover, some proteins did not bind to the First-Gen BLI’s biosensor at all, meaning that it’s not just a question of signal to noise, but whether you see anything at all.

Figure 4: Comparison in binding activity in a typical two-minute binding experiment across twenty different his-tagged proteins on each biosensor.

Biosensor	Avg. signal loaded	vs. Legacy BLI
First-Gen Anti-His	0.17 nm	1x (some showed no binding at all)
Gator Anti-His	0.40 nm	2.4x
Gator HIS XT	1.93 nm	11.3x

That sensitivity comes from two upgrades in parallel: the XT optics themselves, plus a better capture molecule using Monod Bio’s anti-His NovoBody™ technology. Together, Gator’s HIS XT offers an 11.3x signal improvement without a meaningful increase in noise. Importantly: the signal gain from Gator XT biosensors comes by using improved optical physics and capture molecules, not sample rates and smoothing algorithms.

Why isn’t the noise increased as much as signal? A meaningful shift in the interference spectrum comes from a uniform layer of biomolecules across the surface, which produces a clean, consistent change in path length. A major source of noise, by contrast, is transient, non-uniform events at the surface and since these are incoherent, they couple weakly into the interference shift.

This is an important advantage of the XT format: increasing the gain from the biolayer amplifies the true binding signal without proportionally amplifying surface-level noise. The result is a stronger signal-to-noise ratio and better resolution of weak or low-level binding events. We’ll explore this mechanism in greater detail in a future article on the optical physics behind the XT platform.

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Does XT optics allow you to study smaller analytes?

Although noise is incoherent, the signal amplification from XT optics does amplify coherent increases Figure 5 is a dataset with a 2 kDa peptide as analyte on a Gator Pivot instrument which used Peltier cooling to run the 16 channel assay at 20°C, 400 RPM with an SA XT biosensor. The XT sensitivity improvement results in a signal:noise ratio of over 200 – for a 2 kDa analyte!

Figure 5A: Full sensorgram showing binding by 150 kDa dimer as ligand and 2 kDa peptide as analyte. B: Analyte binding and kinetics fitting data. Analyte signal of 0.8 nm shift translates to over 200x signal:noise in a peptide binding experiment. XT optics increases response signal without also increasing noise.

In addition to the added sensitivity for smaller analytes, SA XT biosensors paired improved optics with a unique capture molecule design optimized for larger molecules. On SA XT, we use crosslinked streptavidin on the surface of the biosensor, removing the 3D matrix from the surface and that might hinder larger analytes, like lipid nanodiscs, from penetrating into the biosensor surface. This improved sensitivity and wider compatibility range across both larger and smaller molecules make SA XT a versatile troubleshooting sensor. It became one of our best sellers almost immediately. SA XT biosensors have been cited for numerous applications like nucleic acids [1], AI-driven protein engineering [2], and polysaccharide-protein interactions [3], among others. For more examples, you can search our citations page for Streptavidin XT or SA XT.

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What are some other examples of XT biosensors?

Gator Bio’s XT optics innovation doesn’t increase the number of binding sites on the sensor – it generates more signal per molecule. This means that XT optics can be applied to any capture molecule on the biosensor. Gator has developed many different biosensors that take advantage of XT optics, so whatever your workflow, we likely have a biosensor with XT optics that meets your needs.

Strep-Tactin XT

Gator’s Strep-Tactin XT biosensor uses an engineered Streptavidin molecule, known as Strep-Tactin XT licensed from IBA Lifesciences, as a capture molecule to immobilize twin-strep-tagged proteins (tag sequence: SAWSHPQFEKGGGSGGGSGGSAWSHPQFEK). The twin-strep tag is two copies of the Strep-Tag II sequence (SAWSHPQFEK) which binds to the Strep-Tactin XT tetramer with picomolar affinity, making it an excellent choice for rapid capture and kinetics applications. They also regenerate up to 10 times, making them an excellent choice to run complex, multi-cycle assay formats. Note: proteins tagged with only a single copy of the Strep-tag II sequence will not remain tightly bound to Strep-Tactin XT. Gator offers an anti-Strep-Tag II antibody sensor for molecules that use a single Strep-Tag II tag.

Strep-Tactin XT biosensors originated from our custom biosensor program back in 2023. Built on the XT optical improvements, the Strep-Tactin XT biosensors can capture ligands from cell-free expression media, making them an ideal choice for AI-driven discovery workflows, where the XT optical improvements improve sensitivity for smaller targets. Recent AI-driven discovery and binder-characterization pre-prints have used Strep-Tactin XT in their workflows [4-6]. For more examples, you can search our citations page for Strep-Tactin XT.

Mouse Fc XT (MFC XT)

Mouse Fc XT also represents a significant improvement over traditional Mouse Fc biosensors. Not only does it have the same XT optical improvements, but it also includes a nanobody capture molecule with much faster on-rate and higher affinity, allowing faster screening of hybridoma pools for high throughput kinetics. Because MFC XT gives more signal per molecule, low-expressing clones that produce little or no signal on traditional Mouse Fc sensors often show a clear, positive signal on MFC XT, so you catch weak positives you would otherwise miss. For more information about the MFC XT biosensor, download this application note on using Mouse Fc XT biosensors for hybridoma screening.

Anti-His XT (HIS XT)

We’ve already talked quite a bit about HIS XT biosensors and their development journey in this previous blog post. HIS XT Biosensors combine XT optical improvements with Monod Bio’s new HIS XT NovoBody™, designed using AI to bind to a wide variety of His-tagged proteins with high affinity. HIS XT biosensors are also the best biosensor on the market for high throughput Fc receptor studies for two reasons:

1. The capture molecule has no Fc (and therefore doesn’t cause any Fc interference).
2. The sensitivity improvement derived from the XT optics (more signal per molecule) means that you can load fewer molecules on the surface, thereby improving the accuracy of kinetics experiments on this notoriously difficult interaction pair.

For a detailed protocol on how to use HIS XT for Fc receptor studies, download our application note studying CD32/FcγRIIa interaction here.

Regen SA (RSA)

Regen SA is our newest biosensor, employing an engineered streptavidin surface that can be completely regenerated using a simple two-step process. RSA delivers up to 4x higher signal and cuts cost per interaction by up to 8x versus non-reusable SA sensors. This makes it an ideal choice for high throughput screening for workflows featuring biotinylated molecules or instances where re-using an identical binding surface are important for consistency.

Other biosensors with XT optics

While many of our products are clearly labeled with the XT on the label, several of our other biosensors also incorporate XT optics into their biosensor design. Below is a complete list of biosensors that use Gator XT optics:

In addition to the list above, custom biosensors can be made with XT optics on request. For any questions about our custom biosensors program contact our team.

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Should I approach my experiments differently with XT optics?

The same best practices of BLI still apply. However, XT biosensors give you a leg up in four different ways:

1. Common best practice is to load as few ligand molecules as possible while obtaining a usable amount of signal. With Gator XT optics amplifying signals by 3x, you can load to the same nm shift as you are accustomed to and thereby load 3x fewer ligand molecules onto the surface without affecting analyte binding sensitivity. This can have a dramatic impact for applications that are affected by mass transport or avidity, like Fc receptors (see the His XT application note on Fc receptor studies).
2. Increased sensitivity also gives you flexibility to study analytes down to 2 kDa. Load as much ligand as is needed to observe peptide binding clearly.
3. For picomolar binders, a greater measurable amount of dissociation is helpful for calculating k_off. With an XT biosensor, you can observe slow off rates more clearly because more signal is being removed from the sensor surface through dissociation, even if it’s the same number of molecules. This can improve accuracy of the modeling software as it tries to fit slow-off kinetics.

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How can I compare the difference between XT and standard BLI biosensors in my own lab?

Gator’s XT biosensors are only compatible with Gator instruments, and not with any competitor’s instrumentation. However, if you’d like to see how Gator XT biosensors could improve your data quality, contact us and our team can arrange a demonstration in your lab.

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Works Cited

[1] Lee, C.-C., Hsu, S.-F., Chang, Y.-W., Ho, M.-R., Sugiyama, H., Wang, W.-C., & Wang, A. H.-J. (2026). Structural basis of B-to-Z DNA transition mediated by an anti-Z-DNA antibody. Nucleic Acids Research, 54(5). DOI: 10.1093/nar/gkag160. Full text: academic.oup.com/nar/article/54/5/gkag160

[2] Zhu, Y., Liu, H., Qu, F., Qu, C., Wang, Y., Yao, J., Li, K., Wang, Z., You, S., Hua, L., Ge, C., Yao, H., & Li, T. (2025). De novo binders overcome the MMLV RT stability-activity trade-off. iScience, 28(12), 114140. DOI: 10.1016/j.isci.2025.114140

[3] Xiao, R., Zhou, W., Peng, J., Cai, S., Xie, J., Liao, W., Li, S., Ding, K., & He, F. (2025). AJDW, polysaccharide from Albizia julibrissin, binds to PI3K and induces cell cycle arrest and ROS-mediated pancreatic cancer cell apoptosis. Glycoscience & Therapy, 100003. DOI: 10.1016/j.glycos.2025.100003

[4] Stark, H., Faltings, F., Choi, M., Xie, Y., Hur, E., O’Donnell, T., Bushuiev, A., Uçar, T., Passaro, S., Mao, W., Reveiz, M., Bushuiev, R., Pluskal, T., Sivic, J., Kreis, K., Vahdat, A., et al. (2025). BoltzGen: Toward Universal Binder Design. bioRxiv. DOI: 10.1101/2025.11.20.689494

[5] Du, Y., Yu, B., Liu, T., Shen, T., Chen, J., Rittig, J. G., Sun, K., Zhang, Y., Song, Z., Zhou, B., Masschelein, C., Wang, Y., Wang, H., Jia, H., Zhang, C., Zhao, H., Ester, M., Head-Gordon, T., Gomes, C. P., Sun, H., Duan, C., & Schwaller, P., Jin, W. (2025). Accelerating Scientific Discovery with Autonomous Goal-evolving Agents. arXiv:2512.21782. Link: arxiv.org/abs/2512.21782

[6] Candido, S., Hayes, T., Derry, A., Rao, R., Lin, Z., Verkuil, R., Wu, B. Z., Lee, J. S., Bruguera, E. S., Keval, J. A., Kopylov, M., Pak, J. E., Wu, W., Thomas, N., Mataraso, S., Rives, A., et al. (2026). Language Modeling Materializes a World Model of Protein Biology. bioRxiv. DOI: 10.64898/2026.06.03.729735