One-step rapid syphilis test detects infection earlier, researchers report

Penicillin has cured syphilis cases since 1943, but symptoms for the sexually transmitted infection, known as "the great imitator," mimic many other diseases and can come and go without treatment while the infection keeps causing harm. If it remains undetected, it can spread to partners and to babies during pregnancy, causing lifelong effects. A new rapid test developed by a team led by Penn State researchers detects lower levels of the two types of antibodies the body makes to fight the syphilis bacteria and respond to disease-related cell damage, potentially leading to earlier interventions and fewer appointments for patients.

Key to the advancement are the nanoparticles that accumulate and form the visible line for a positive result on most diagnostic tests, according to corresponding author Dipanjan Pan, Penn State’s Dorothy Foehr Huck & J. Lloyd Huck Chair Professor in Nanomedicine. The team’s new pilot testing device, published in a study in ACS Sensors, swapped the typical gold nanoparticle for one layered in silver and sealed in more gold to produce test lines 19% to 46% stronger than those of comparable devices while reliably detecting lower concentrations of antibodies.

“The low-titer range, meaning the range with the lowest concentration of antibodies, is particularly challenging for detecting syphilis,” Pan said. He added that nanoparticles accumulate into a visible line as they bind to more and more antibodies in a sample flowing through the device. “This range is also clinically important, occurring during early infection, later stages of syphilis or after treatment when antibody levels are declining to make sure treatment worked.”

In the diagnostic pathway established by the U.S. Centers for Disease Control and Prevention (CDC), the first “treponemal” blood test detects antibodies against the syphilis bacteria, Treponema pallidum. These antibodies stay in the body for decades, and a patient who received treatment years ago will still test positive as long as the antibodies remain in their system. The second “non-treponemal test” looks for separate antibodies created in response to cellular damage, indicating the stage of infection and whether a person needs treatment.

Running both tests lessens the chance for false-negative or false-positive results and gives doctors a better sense for whether the disease occurred in the past, is currently active or has been properly treated, according to the CDC.

“What is important about our approach is that we are not asking clinicians to abandon this well-established diagnostic pathway,” Pan said. “Instead, we are trying to reproduce the same sequence of diagnostic information in a much faster and potentially point-of-care format.”

The limitation to this point, he explained, was not the ability to detect the antibodies but the ability for clinicians to see the gold nanoparticles that only become visible to the naked eye when enough of them accumulate. One way to make them “brighter” was to increase their size. However, around 60 nanometers, roughly 100 times smaller than a red blood cell, they start clumping or settling like sand in a river rather than flowing through the device.

The team instead altered the nanoparticle’s composition. According to Pan, particles behave differently on the nanoscale, and the electrons of gold atoms in nanoparticles interact collectively and absorb wavelengths of light differently. This gives them the ruby-red color commonly seen in rapid tests instead of gold’s characteristic hue.

“At the nanoscale, I like to picture the electrons on the particle’s surface as the ripple of standing and sitting that happens when a stadium full of people participate in ‘the wave,’” said first author Ketan Dighe, a Penn State biomedical engineering graduate student. “That collective motion is called localized surface plasmon resonance, and when light hits them, it’s what gives the particles their color and makes them so useful for detection.”

When a particle has multiple metallic layers, Dighe added, the electrons that would typically only interact with other electrons from the same layer, or “plasmon,” start to collectively combine features across the metallic boundaries, creating a much stronger optical signal. The team sandwiched a layer of silver, which has stronger optical properties than gold but degrades quickly when exposed to oxygen, between two layers of gold. This kept the particle stable while taking advantage of the silver and the resulting combined features from multiple metallic layers.

Pan likened the resulting stronger optical properties to another collective human activity: a choir.

“One singer can be heard, but when several singers are carefully coordinated, their voices emphasize one another and the sound becomes much more powerful,” Pan said. Those three layers work together to produce a much stronger optical signal than the original gold particle alone.”

Prior to embedding the nanoparticles in a lateral flow reporter — the typical set up of at-home detection tests, like those used for pregnancy or COVID-19 — the team first compared the optical performance of the multilayer nanoparticles to conventional gold. In these laboratory benchmark tests, the team found six times fewer multilayer particles were needed to form a visible mark than gold alone. The multilayer particles also remained visible with three times fewer sample target molecules, an encouraging sign that they would be able to detect syphilis antibodies in low concentrations, according to Pan.

The team then tested the nanoparticles using 36 clinical specimens spanning a range of antibody concentrations from the CDC and Boca Biolistics, a commercial biorepository. In these experiments, line intensities increased 19% to 46% compared to conventional gold nanoparticles. The largest gains were found in tests detecting antibodies in the lowest concentrations.

In practice, that sensitivity would give healthcare professionals vital information to determine treatment options before a patient leaves the clinic, according to co-investigator Casey Pinto, a clinician and associate professor of public health sciences at Penn State College of Medicine.

“If I could tell a patient what infection they had during a visit, the counseling, the information I provide and the treatment would dramatically change,” Pinto said.

Nearly 4,000 babies in the U.S were born with syphilis in 2024, the 12th consecutive annual increase. The CDC reported that a lack of timely testing and adequate treatment contributed to nearly 90% of these congenital cases.

“Syphilis is one of those unique bacteria that can impact every single organ of the body,” Pinto said. “If we do not catch syphilis and the woman is pregnant, it could infect the child, too. The baby can be born with developmental delays, heart problems, blindness, teeth and bone malformations, skin rashes and it can even lead to death.”

Two rapid tests for syphilis have been approved by the U.S. Food and Drug Administration in recent years, but they only detect treponemal antibodies. According to Pan, there are currently no single-step confirmatory syphilis tests approved for use in the U.S. One exists in Europe, but it does not detect antibodies in the low-titer range.

He said the team applied for a patent for the device and is exploring opportunities to partner with industry as they move towards larger validation studies, further prototype development and regulatory submission.

“We could potentially move syphilis testing from a multistep laboratory process toward a model of testing, diagnosing and treating in a single clinical visit,” he said.

Reference:

Ketan Dighe, Satheesh Natarajan, Maria Iftesum, Shraddha Krishnakumar, David Skrodzki, Matthew Molinaro, Nivetha Gunaseelan, Casey N. Pinto, Manas Ranjan Gartia, Dipanjan Pan; Multilayer Plasmon Hybridization in Au–Ag–Au Nanoparticles Enables Extinction Amplification in Transport-Limited One-Step Confirmatory Syphilis Biosensing. ACS Sens. 2026; https://doi.org/10.1021/acssensors.6c02500