New Nanoparticle Helps Rapid Syphilis Tests Catch Low-Level Infections

Gold and Silver Particle Sharpens Rapid Syphilis Test Signals

A modern rapid syphilis test can miss an infection when antibody levels in the blood run low, but a new particle built from layers of gold, silver, and gold may close that gap. In laboratory tests on clinical blood samples, strips carrying the layered particles produced test lines 19% to 46% stronger than strips with ordinary gold particles, and picked up low-level samples that the gold strips failed to show, according to a study in ACS Sensors.

Faint antibody signals matter most in early infection and during pregnancy, when untreated syphilis is linked to stillbirth, premature birth, and passing the infection to the baby. Cases, especially in newborns, have climbed sharply over the past decade, according to the paper. Rapid strips are meant to give an answer in a single visit, since sending samples to a lab adds delays, and some patients are lost before follow-up.

Researchers at Penn State and Louisiana State University set out to brighten that signal without making the particles too big to travel through the strip. Their strips develop in about 10 minutes, and the lines can be read by eye.

Plain Gold Particles Leave a Rapid Syphilis Test Too Faint

Rapid syphilis strips work much like a home pregnancy test. A sample creeps along a paper-like strip, and antibodies in it latch onto tiny gold particles, which pile up at a test line and turn it dark. This strip has two lines. One flags antibodies that usually stay detectable for life after infection, while the other tracks antibodies that fade after successful treatment, helping tell active infection from past infection.

Today, confirming syphilis usually takes a chain of lab tests. Rapid options exist, but the paper notes that some check for only one kind of antibody, and others lose sensitivity when antibody levels are low. Plain gold particles are part of the reason. With few antibodies around, too few particles reach a line to make it visible.

Bigger particles give a stronger color, but past roughly 60 nanometers, or 60 billionths of a meter, they struggle to squeeze through the strip’s fibers. For scale, a typical human hair is more than 1,000 times wider. Gold particles are stuck with a trade-off: small enough to travel, but too faint when antibodies are scarce.

A Silver Middle Layer Makes Gold Particles Easier to See

Researchers started with a gold particle, wrapped it in a thin layer of silver, then added an outer shell of gold. Microscope images confirmed the silver sits between the two gold layers, like the filling in a sandwich, and the finished particles stayed small enough to travel.

Silver reacts to light far more strongly than gold, but it tarnishes easily and is harder to attach antibodies to. The gold coating shields it and keeps the particles stable in liquid. Computer simulations of an idealized version showed up to 18 times more light absorption than gold particles of the same size. In lab tests, a visible spot on the strip material needed about six times fewer layered particles than plain gold ones, and the layered spot came out a deeper, brownish red than gold’s plain red.

With a stand-in protein, layered particles also detected one-third as much material as plain gold could, a threefold gain. The gold comparison particles were nearly the same size, so researchers credit the gains mainly to the layering, not to bulk.

Syphilis Test Strips Matched Lab Results in 36 Samples

Next came clinical samples. Researchers tested 36 de-identified serum samples, the liquid part of blood, supplied by the CDC and a commercial supplier, Boca Biolistics. Most came from people with confirmed syphilis, a few from healthy controls, and antibody levels ranged from very low to very high.

Results matched standard laboratory tests in every sample, and the biggest gains showed up on the faintest ones.

Four samples carried only the lifelong antibodies, and the strip showed a signal on just that line, a hint that the two-line design can separate past or treated infection from active disease. Two HIV samples and two hepatitis C samples produced no false lines, though four samples is a small set for ruling out false alarms from other infections.

Fingerstick testing remains a goal, not a result. Every sample was serum, so the version that works from a finger prick, with plasma separated right on the strip, is still future work, along with testing in more places and more varied groups. If it pans out, strips like these could let clinics test and treat in the same visit, especially where lab access is limited. Using fewer particles could eventually lower costs, but the paper reports no cost figures. The authors describe the layering as a general design approach that could apply to other rapid tests.

A rapid test earns its speed only if it catches the infections that are hardest to see, and gold strips struggled with exactly those. Layered particles brought faint samples into view in the lab. Until clinic trials confirm it, the gain remains a lab result, not a proven one.

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Disclaimer: This article reports on a single laboratory study and is for informational purposes only. It is not medical advice and does not recommend any test or treatment. The test described was evaluated only in the lab. Anyone concerned about syphilis or another infection should talk with a qualified health care professional.

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