Comparative performance of three optical biosensing platforms for SARS-CoV-2 antibodies detection in human serum

This study rigorously compares Bloch surface wave (BSW) and microring resonator (MRR) optical biosensing platforms, demonstrating that both label-free technologies offer rapid, sensitive, and reproducible detection of SARS-CoV-2 antibodies in human serum, positioning them as promising candidates for next-generation clinical diagnostics.

The Gist

Imagine you are a detective trying to solve a mystery: Did a person's immune system fight off the SARS-CoV-2 virus?

To do this, you need to find "wanted posters" (antibodies) left behind in their blood. For a long time, the gold standard for finding these posters was a heavy, expensive, and slow machine called SPR (Surface Plasmon Resonance). It's like using a massive, high-tech metal detector to find a single lost earring.

This paper introduces two new, sleeker detectives: BSW (Bloch Surface Wave) and MRR (Microring Resonator). The researchers wanted to know: Can these new detectives find the antibodies just as well as the old heavy machine, but faster and cheaper?

Here is the story of how they compared them, explained simply.

1. The Setup: A Race on the Same Track

Usually, comparing two different machines is like comparing a Ferrari and a bicycle by racing them on different tracks. One might be on a highway, the other on a dirt road. To make it fair, the scientists built a brand-new BSW machine right next to the existing MRR machine in the same lab. They used the exact same blood samples, the exact same weather (room temperature), and the exact same rules.

2. The Tools: How They "See" the Antibodies

Both new machines use light, but they catch the antibodies in different ways:

• The BSW (The "Dielectric Mirror"):
  Imagine a stack of perfectly clear glass and plastic layers, like a very fancy sandwich. When you shine a laser at a specific angle, the light gets "trapped" and skims along the surface, like a stone skipping on water.

  • The Magic: When an antibody lands on this surface, it changes the "skipping" angle of the light. The machine sees this tiny shift and says, "Aha! An antibody is here!"
  • The Analogy: Think of it like a trampoline. If you jump on it alone, you bounce a certain way. If someone else jumps on with you, the bounce changes. The machine feels that change.

• The MRR (The "Light Loop"):
  Imagine a tiny, invisible race track for light (a ring) carved into a silicon chip. Light runs around this loop over and over.

  • The Magic: The light only stays in the loop if it's running at a very specific speed (wavelength). When an antibody sticks to the track, it makes the track slightly "heavier" (changing the refractive index), forcing the light to slow down or speed up to stay in sync. The machine detects this speed change.
  • The Analogy: Imagine a runner on a circular track. If you suddenly put a heavy backpack on the runner, they have to change their stride to keep running. The machine notices the stride change.

3. The Test: The Blood Samples

The researchers tested blood from two real people:

• Person A (S404): Got vaccinated but never got sick. Their blood should be full of antibodies against the "Spike" protein (the vaccine target) but empty of antibodies against the "Nucleocapsid" (a part of the virus only present if you were infected).
• Person B (S405): Got infected before getting vaccinated. Their blood should be full of antibodies against both the Spike and the Nucleocapsid.

The Result: Both the BSW and MRR machines got the story right. They correctly identified who had been vaccinated and who had been infected, matching the results of the expensive "gold standard" machines.

4. The Twist: The "Sticky" Problem

Here is where the story gets interesting. The researchers tried to clean the machines to use them again (like washing a plate).

• The Nucleocapsid Test: When they tested for the Nucleocapsid antibodies, the antibodies let go easily when washed. The machine could be cleaned and reused.
• The Spike Test: When they tested for the Spike antibodies, the antibodies were too sticky. They held on so tight that even a strong cleaning solution couldn't wash them off. The machine got "clogged" and couldn't be reused for that specific test.

The Metaphor: Imagine trying to wash a sticker off a window.

• The Nucleocapsid sticker was like a piece of tape; you could peel it off and wash the window to use it again.
• The Spike sticker was like superglue. Once it was on, it was permanent. You had to throw the window away (or use a fresh one) for the next test.

5. The Verdict: What Does This Mean?

The paper concludes that both BSW and MRR are fantastic new tools.

• They are fast: They give results in minutes, not hours.
• They are cheap: They don't need expensive metal parts or complex fluids; they are essentially plastic and glass.
• They are accurate: They match the results of the big, expensive hospital machines.

The Big Picture:
Think of the current medical testing landscape as a world where only a few people have a Ferrari (the big SPR machines) to check for viruses. This paper shows that we can now build bicycles (BSW and MRR) that are just as good at getting to the destination.

This means we could eventually have cheap, disposable "test cards" that anyone can use to check their immune status, helping us track outbreaks and see if vaccines are working, without needing a massive laboratory. It's a step toward making high-tech health monitoring accessible to everyone.

Technical Summary

1. Problem Statement

The emergence of SARS-CoV-2 highlighted a critical need for advanced serological tools to assess immune responses, vaccine efficacy, and herd immunity. While RT-PCR is the gold standard for active infection, serological assays are required for antibody quantification. Current methods often rely on secondary labels (e.g., fluorescence) or expensive, bulky instrumentation (e.g., commercial Surface Plasmon Resonance, SPR). There is a need for label-free, rapid, quantitative, and low-cost optical biosensing platforms that can be integrated into disposable formats for clinical diagnostics and surveillance.

2. Methodology

The study conducted a rigorous comparative analysis of three optical biosensing platforms under nearly identical experimental conditions to eliminate inter-laboratory variability:

• Platforms Compared:

  1. Bloch Surface Wave (BSW): A label-free platform using a purely dielectric 1D photonic crystal (1DPC) stack ($SiO_2/Ta_2O_5/TiO_2$) on a glass substrate. It operates via angularly resolved reflectance detection at $\lambda = 632.8$ nm.
  2. Microring Resonator (MRR): A silicon nitride waveguide-based platform using ring resonators (165 $\mu$m diameter) coupled to a bus waveguide. Detection relies on resonant wavelength shifts.
  3. Surface Plasmon Resonance (SPR): Used as a benchmark (Cytiva Biacore X100) with a standard dextran surface (CM5 chip).

• Experimental Setup:

  • A new BSW readout apparatus was established at the University of Rochester Medical Center on the same optical table as the existing MRR platform.
  • Samples: Human serum samples from two donors (S404 and S405) with known vaccination and infection histories (confirmed by ZIVA Arrayed Imaging Reflectometry scores).
  • Probes: Biosensor surfaces were functionalized with various SARS-CoV-2 antigens:
    • Spike protein variants: Wild-type (S-wt), Omicron (S-O), BA.5 (S-BA.5).
    • Nucleocapsid protein (N-wt).
    • Negative controls: Anti-fluorescein (a-FITC) or ethanolamine-blocked surfaces.
  • Assay Protocol: Label-free detection of antibodies in diluted human serum. The study included regeneration tests (reusability) and kinetic analysis.

3. Key Contributions

• Direct Comparative Analysis: Unlike previous studies that compared platforms across different labs, this work established a new BSW system adjacent to an MRR system, allowing assays to be performed with the same serum samples, operators, and handling procedures.
• Label-Free Detection in Complex Media: Demonstrated that both BSW and MRR can detect SARS-CoV-2 antibodies directly in human serum without secondary labels, overcoming the challenges of non-specific binding in high-protein environments.
• Kinetic Modeling: Validated a bivalent binding kinetic model for the interaction between serum antibodies and immobilized antigens, showing a linear dependency between the initial binding slope and serum concentration.
• Regeneration Feasibility: Investigated the reusability of BSW biochips, identifying distinct behaviors based on the captured antigen (Spike vs. Nucleocapsid).

4. Key Results

• Performance Consistency:

  • Both BSW and MRR platforms successfully detected anti-Spike and anti-Nucleocapsid antibodies.
  • Results were highly consistent with the longitudinal serology benchmarks (ZIVA scores) and with each other.
  • BSW Specifics: Showed excellent batch-to-batch repeatability across different biochips. The resonance shift ($\Delta\theta$) clearly distinguished between high anti-Spike/low anti-Nucleocapsid (Sample S404) and high anti-Nucleocapsid/low anti-Spike (Sample S405) profiles.
  • MRR Specifics: Showed comparable sensitivity, though with a residual non-specific signal (~100 pm) that was not fully corrected by the negative control.

• Regeneration and Reusability (BSW):

  • Nucleocapsid (N-wt): The biochip surface could be successfully regenerated using a low-pH glycine/citric acid solution, allowing for multiple assay cycles.
  • Spike (S-wt): Regeneration failed; the high affinity of anti-Spike antibodies prevented unbinding, leading to surface saturation. Consequently, S-wt functionalized chips are single-use, while N-wt chips are reusable.

• Kinetic Analysis:

  • The initial binding kinetics followed a linear dependency on serum dilution, consistent with a bivalent binding model ($A + L \rightleftharpoons AL \rightleftharpoons AL_2$).
  • Time Constants: While microfluidic differences (active flow in BSW/SPR vs. passive capillary in MRR) affected absolute values, the primary binding time constants were comparable across all three platforms (ranging from ~1 to 7.5 minutes depending on the dominant kinetic component).
  • SPR Benchmark: SPR characterization of monoclonal antibodies (a-BA.5) against S-BA.5 confirmed a bivalent binding model was necessary for accurate fitting, yielding a dissociation constant ($K_D$) in the nanomolar range ($6.15 \times 10^{-9}$ M).

• Sensitivity and Resolution:

  • BSW demonstrated extremely narrow resonance widths (angularly resolved), offering high resolution comparable to SPR but without metal-induced quenching.
  • MRR provided rapid detection via passive flow, suitable for point-of-care applications.

5. Significance

This study establishes BSW and MRR technologies as powerful, low-cost candidates for next-generation clinical diagnostics.

• Clinical Utility: Both platforms offer rapid, quantitative, and label-free detection suitable for serological surveillance and vaccine monitoring.
• Scalability: The ability to integrate these sensors into disposable, injection-molded formats (especially MRR) and the high repeatability of BSW biochips suggest a pathway toward mass-deployable diagnostic tools.
• Differentiation: The study highlights that while both technologies are effective, their specific fluidic and regeneration characteristics dictate their optimal use cases (e.g., MRR for disposable, passive-flow screening; BSW for high-precision, potentially reusable Nucleocapsid assays).

In conclusion, the work validates that dielectric photonic crystal (BSW) and silicon photonic (MRR) sensors can match the performance of established SPR systems for SARS-CoV-2 serology, offering a viable alternative for future pandemic preparedness and routine immune monitoring.