Ricin, a highly toxic glycoprotein derived from the seeds of Ricinus communis, poses significant risks in bioterrorism and toxicology due to its rapid absorption and ease of dissemination. Rapid, ultra-sensitive detection is crucial for timely medical intervention and implementing security measures. However, existing methods often lack sufficient sensitivity or require lengthy processing, limiting their utility for trigger-to-treat scenarios. Here, we present an optical modulation biosensing (OMB)-based ricin assay capable of detecting low concentrations of ricin in buffer, plasma, and biological samples. The assay combines magnetic-bead-based target capture with fluorescent signal enhancement, achieving a limit of detection (LoD) of 15 pg/mL in buffer and 62 pg/mL in plasma, with a 4-log dynamic range. Optimized protocols reduced the assay time to 60 min, maintaining an LoD of 114 pg/mL in plasma while preserving accuracy and reproducibility. The assay successfully detected ricin in bronchoalveolar lavage fluid and serum from mice that were intranasally exposed to ricin, with signals persisting up to 48 h post exposure. Its rapid, high-throughput capabilities and simplified workflow make the OMB-based assay a powerful tool for toxicology, forensic analysis, and counter-bioterrorism. This study highlights the OMB platform’s potential as a sensitive and robust diagnostic tool for detecting hazardous biological agents.

Rapid, sensitive, and high-throughput detection of biomarkers at low concentrations is crucial for early disease diagnosis. Many sensitive immunoassays use magnetic beads to capture fluorescently labeled targets, but quantifying these targets involves detecting the fluorescent signal from individual beads, which is time-consuming and requires a costly detection system. Additionally, there is often a trade-off between sensitivity, speed, throughput, and ease of use. A new technology, high-throughput optical modulation biosensing (OMB), enables reading a 96-well plate within 10 minutes. In OMB, a cylindrical permanent magnet immobilizes the magnetic beads at the illumination spot. Then, a laser beam is manipulated between the magnetic beads cluster and the background solution, effectively subtracting noise and reducing the need for washing and separation steps, which are usually incorporated in heterogeneous assays. This technology has evolved into a fully automated platform with high sensitivity and throughput, allowing much faster turnaround time and better sensitivity than the state-of-the-art methods, like enzyme-linked immunosorbent assay (ELISA) (for protein detection) and real-time PCR (for RNA/DNA detection). Here, we provide a comprehensive review of this technology, its development, and its applications in rapid, highly sensitive detection of proteins (e.g., human Interleukin-8) and viruses (e.g., SARS-CoV-2).

Fluorescence-based immunoassays are widely used for sensitive biomarker detection, but their performance at low analyte concentrations is often limited by background signals originating from unbound fluorophores, Raman scattering, and sample autofluorescence. Here, we introduce chopped optical biosensing (COB), an analytical framework that improves fluorescence immunoassay sensitivity by temporally separating background and target fluorescence. In COB, fluorophore-labeled targets captured on magnetic beads are concentrated into a small detection region using a permanent magnet. An optical chopper periodically gates the excitation beam, enabling fluorescence measurements before bead aggregation (background only) and after aggregation (target + background). Subtraction of these temporally separated signals isolates fluorescence from bead-bound targets while suppressing background contributions from unbound fluorophores, Raman scattering, and sample autofluorescence. By limiting continuous illumination, this approach also reduces photobleaching and stabilizes excitation intensity, while eliminating the need for washing steps or dynamic optical modulation. Analytical evaluation demonstrated detection limits of 61 fM for Atto 532 dye, 0.05 ng/L for interleukin-8 (IL-8), and 3 ng/mL for anti-dengue virus serotype 2 non-structural protein 1 (anti-DENV-2 NS1) immunoglobulin G (IgG), with reproducibility below 15%. The total assay time is approximately 90 min. In a targeted feasibility assessment using 64 well-characterized serum samples, the COB-based anti-DENV-2 NS1 IgG assay correctly classified all samples when compared with reference ELISA results. Combining low complexity, compact design, and energy-efficient operation with high analytical and clinical performance, COB provides a practical foundation for streamlined fluorescence immunoassay workflows and is well suited for future portable diagnostic applications.

Microfluidic fluorescence immunoassays enable sensitive biomarker detection, but performance is limited by inefficient capture on planar surfaces and background fluorescence from unbound labels. Here, we present chip-based optical modulation biosensing (Chip-OMB), combining three-dimensional magnetic bead-based target capture, bead immobilization, internal fluid flushing, and modulated optical background subtraction. Magnetic beads are concentrated and immobilized in the chamber, while controlled on-chip flushing displaces unbound fluorescent labels without requiring user-performed washing after chip loading. Alternating optical interrogation of the bead cluster and surrounding solution enables background subtraction. Using fluorescently labeled magnetic beads, Chip-OMB achieved limits of detection of 0.1 pM in buffer and 22 pM in undiluted plasma, corresponding to 580-fold and 22-fold lower equivalent fluorophore-concentration limits of detection, respectively, than conventional steady-state bulk fluorescence measurements using a plate reader. Measurements correlated with the laboratory optical modulation biosensing system (R² = 0.98). In an anti-dengue virus serotype 2 (DENV-2) nonstructural protein 1 (NS1) immunoglobulin G (IgG) immunoassay, Chip-OMB achieved a 3 ng/mL limit of detection with a ~4-log dynamic range. Among 56 sera, 29 DENV-2 cases confirmed by reverse transcription quantitative polymerase chain reaction (RT-qPCR) were Panbio IgG-positive and above the exploratory Chip-OMB cutoff, while all 27 study-defined negative controls were below it.

We present the calculations of the Curie temperature and magnetization of doped EuO both in the absence and in the presence of external magnetic field. The calculations were performed both for the free electrons model and for the model with finite electron band width. Both models give similar results for the magnetization, close to Brillouin function.