A Penn State-led team developed a rapid test that detects two types of syphilis antibodies in a single step, with improved sensitivity when antibody levels are low.
Researchers led by Pennsylvania State University have developed a pilot rapid diagnostic test that integrates two clinical blood tests into a single device, enabling earlier detection of syphilis at low antibody concentrations.
Detailed in a study published in ACS Sensors, the platform replaces standard gold nanoparticles with multilayer nanoparticles composed of a silver core sandwiched between two protective gold layers. This structure produces test lines that are 19% to 46% stronger than conventional lateral flow assays while reliably identifying both treponemal and non-treponemal antibodies in low-titer samples.
Syphilis symptoms often mimic other conditions and can disappear without treatment while systemic damage progresses. Left untreated, the infection can be transmitted to sexual partners or passed to fetuses during pregnancy, leading to severe congenital complications.
“The low-titer range, meaning the range with the lowest concentration of antibodies, is particularly challenging for detecting syphilis,” says Dipanjan Pan, Dorothy Foehr Huck and J. Lloyd Huck chair professor in nanomedicine at Penn State and corresponding author of the study, in a release. “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.”
Preserving the CDC Diagnostic Algorithm
Standard testing guidelines established by the US Centers for Disease Control and Prevention (CDC) rely on a two-step laboratory pathway. The first test, a treponemal assay, detects antibodies specific to Treponema pallidum, which typically remain detectable in the body for decades, even following curative therapy. The second, a non-treponemal test, identifies antibodies generated in response to cellular damage, reflecting active infection, disease staging, and therapeutic response.
While two rapid syphilis tests have received clearance from the US Food and Drug Administration in recent years, they detect only treponemal antibodies and cannot distinguish active from past treated infections. Currently, no single-step confirmatory syphilis tests are approved for clinical use in the US, and an existing European test does not detect antibodies in the low-titer range.
“What is important about our approach is that we are not asking clinicians to abandon this well-established diagnostic pathway,” says Pan in a release. “Instead, we are trying to reproduce the same sequence of diagnostic information in a much faster and potentially point-of-care format.”
Engineering Stronger Optical Signals at the Nanoscale
Conventional lateral flow tests depend on gold nanoparticles accumulating to create a visible signal. However, increasing particle diameter beyond roughly 60 nanometers causes agglomeration and settling in fluid channels rather than smooth migration across the membrane.
To overcome this physical constraint, the engineering team altered particle architecture using localized surface plasmon resonance. By encasing an optically active silver layer inside gold, the researchers prevented oxidative degradation while allowing electrons across the metallic boundaries to couple, generating a substantially stronger optical signal.
“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,’” says Ketan Dighe, graduate student in biomedical engineering at Penn State and first author of the study, in a release. “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.”
In laboratory benchmark evaluations, six times fewer multilayer particles were needed to produce a visible signal compared to gold nanoparticles alone, and the engineered particles remained visible with three times fewer target molecules.
The investigators subsequently validated the platform using 36 clinical specimens provided by the CDC and Boca Biolistics across variable antibody concentrations. Test lines showed 19% to 46% intensity increases compared to standard gold nanoparticles, with the most pronounced gains observed in specimens containing the lowest antibody titers.
Addressing Congenital Syphilis Surges
Improving analytical sensitivity in point-of-care testing carries major public health implications. In 2024, nearly 4,000 infants in the US were born with congenital syphilis, marking the 12th consecutive annual increase. According to CDC surveillance data, lack of timely testing and adequate treatment contributed to nearly 90% of those cases.
“Syphilis is one of those unique bacteria that can impact every single organ of the body,” says Casey Pinto, PhD, associate professor of public health sciences at Penn State College of Medicine, and study co-investigator, in a release. “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.”
Pinto noted that same-day confirmatory results could allow providers to deliver immediate counseling, education, and stage-appropriate treatment during a single patient encounter.
The research team has submitted a patent application for the technology and is seeking industry partners to initiate broader clinical validation studies, complete prototype refinement, and pursue regulatory clearance.
Photo caption: A new pilot rapid test for syphilis from a team led by Penn State researchers integrates two blood tests into one and detects bacteria at very low levels to detect the disease in its earliest stages.
Photo credit: Pan Research Lab
