Ultra‑Sensitive Magnetoelastic Immunosensor for Carcinoembryonic Antigen Detection

Abstract

We report a wireless magnetoelastic immunosensor that achieves unprecedented sensitivity for carcinoembryonic antigen (CEA) detection. By optimizing micro‑chip dimensions through simulation and experimentation, and leveraging gold nanoparticle (AuNP) amplification together with a biocompatible self‑assembled monolayer (SAM), the sensor demonstrates a stable, linear response across a logarithmic CEA concentration range of 0.1–100 ng/mL, with a detection limit of 2.5 pg/mL. The device’s design ensures excellent reproducibility, stability, and rapid response, making it a promising tool for early cancer diagnostics.

Background

Malignancies remain the leading cause of mortality worldwide, with early detection offering the best chance for successful intervention [1]. Tumor biomarkers, such as carcinoembryonic antigen (CEA), become clinically relevant when their serum levels rise above baseline thresholds [2]. CEA, a glycoprotein weighing 180–200 kDa, is typically undetectable (<5 ng/mL) in healthy adults but ascends markedly in colorectal, gastric, pancreatic, lung, and breast cancers [3–7]. Monitoring its concentration allows clinicians to screen, diagnose, and track treatment response, with sensitivity exceeding 80% for recurrence detection [8]. Biosensors that translate molecular recognition into measurable signals offer rapid, cost‑effective, and highly specific analyses. Immunosensors—particularly those employing enzymatic, fluorescent, or electrochemical readouts—have shown remarkable performance for low‑concentration tumor markers [9–11]. Nanoparticle (NP) integration further enhances signal transduction through unique optical, electronic, or magnetic properties [12–14]. Magnetoelastic (ME) sensors, unaffected by ambient temperature or pH, provide high‑sensitivity mechanical readouts and are ideally suited for label‑free detection [15]. In this study, we combine AuNPs with ME micro‑chips to construct a wireless, ultra‑sensitive CEA immunosensor.

Results and Discussion

Due to the ribbon‑like geometry of the ME micro‑chip, magnetic permeability peaks along its length [16]. Initial trials identified 1 mm width and 28 µm thickness as optimal dimensions [17]. Finite‑element simulation further refined the chip length, revealing maximum relative displacement—and thus theoretical sensitivity—at 6 mm (Figure 1). Consequently, the final chip dimensions were 6 mm × 1 mm × 28 µm. Figure 2 illustrates the sensor architecture. The chip surface was first functionalized with cysteine‑derived SAMs, creating a platform for covalent attachment of CEA antibodies (CEAAb). Bovine serum albumin (BSA) was subsequently introduced to mitigate nonspecific binding and steric hindrance. Atomic force microscopy (AFM) confirmed a SAM thickness of 120 nm (Figure 3a). Progressive roughness in AFM images (Figures 3b,c) verified antibody immobilization and CEA capture, with captured complexes measuring ~200 nm in height. The antibody concentration critically influences sensitivity. Testing CEAAb at 20, 50, 70, and 100 µg/mL revealed peak response at 50 µg/mL, with higher concentrations causing steric hindrance and electrostatic repulsion (Figure 4). Real‑time frequency monitoring (Figure 5a) showed a stable response after 40 min, and the frequency shift correlated linearly with log‑transformed CEA concentrations (R² = 0.9688) over 0.1–100 ng/mL (Figure 5b). The calculated detection limit of 2.5 pg/mL surpasses previously reported ME‑based platforms [18]. Overall, the integration of AuNPs and BSA with ME micro‑chips yields a wireless immunosensor capable of ultra‑sensitive, reproducible CEA detection, offering significant advantages for non‑invasive cancer screening.

Conclusions

We have developed a Nano‑ME immunosensor that delivers high‑sensitivity, low‑limit detection of CEA (2.5 pg/mL) across a wide dynamic range (0.1–100 ng/mL). The combination of AuNP amplification and BSA blocking enhances both sensitivity and stability. Given its specificity, ease of use, and reproducibility, this platform holds promise for clinical adoption in early cancer detection and monitoring.

Methods

The ME micro‑chip vibrates longitudinally under a time‑varying magnetic field. The resonance frequency f₀ is governed by

\[\displaystyle f_0 = \frac{1}{2L}\sqrt{\frac{E}{\rho(1-\nu^2)}}\]

where E is Young’s modulus, ν is Poisson’s ratio, ρ is density, and L is chip length. When environmental conditions remain constant, the frequency shift Δf relates to added mass Δm via

\[\displaystyle \frac{\Delta f}{\Delta m} = -\frac{f_0}{2M}\]

Thus, binding of target molecules reduces the resonance frequency proportionally to the mass of bound CEA. Smaller chip dimensions lower the initial mass M, yielding higher sensitivity. ME alloy (Metglas 2826MB, Fe₄₀Ni₃₈Mo₄B₁₈) was laser‑cut into 6 mm × 1 mm × 28 µm chips, cleaned sequentially with acetone, isopropanol, ethanol, and deionized water, then plasma‑activated. A 100 nm chromium seed layer was sputtered, followed by 40 nm AuNP coating. The chips were treated with 40 mM cysteamine in 99% BSA PBS for 12 h at room temperature, then incubated with 50 µg/mL CEAAb activated by 10 mg/mL EDC and 10 mg/mL NHS for 1 h at 37 °C. Final blocking with 0.1% BSA lasted 30 min. For sensing, the chip was housed in a glass tube wound with a coil, connected to a vector network analyzer. An alternating magnetic field excited the chip, and resonance frequency was recorded every 5 min for 40 min upon addition of CEA (0–100 ng/mL). Post‑measurement, chips were rinsed with PBS and imaged by AFM.

Abbreviations

AFM:

Atomic Force Microscope

AuNPs:

Gold Nanoparticles

BSA:

Bovine Serum Albumin

CEA:

Carcinoembryonic Antigen

CEAAb:

CEA Antibody

EDC:

1‑Ethyl‑3‑Carbodiimide

Hz:

Frequency

ME:

Magnetoelastic

NHS:

1‑N‑Hydroxysulfosuccinimide

PBS:

Phosphate‑Buffered Saline

SAM :

Self‑Assembled Monolayer