A tandem giant magnetoresistance assay for one-shot quantification of clinically relevant concentrations of N-terminal pro-B-type natriuretic peptide in human blood

Figures

• 

  Fig. 1. The assay protocol. a Surface modification. Sensor I and II represent the different GMR sensors, and Ab1 and Ab2 represent the respective capture antibodies. b Sample loading and detection antibody binding. Yellow dots represent the biotin tags for binding to the avidin-coated magnetic nanoparticles. c Analyte binding. d Magnetic particle binding and detection

• 

  Fig. 2. The assay system used in the experiments. a Schematic drawing of the chip layout [23]. The deposition areas for avidin-coated magnetic particles (MP) and biotinylated detection antibodies (Ab) are marked. Wc, waste cavity; Wac, washing buffer cavity; Sc, sample cavity; MPc, magnetic nanoparticle rehydration buffer cavity; Rc, reaction cavity. b POCT cartridge, consisting of the microfluidic sample handling circuitry with on-chip cavities, the sensor array chip, and a contact pad interface. c GMR sensor array. The two selected sensor units SI and SII used in tandem are marked in yellow. Two uncoated sensors (TC) used for calculating a compensating factor to account for temperature changes during the measurement are marked in red. d Magnification of a single sensor unit, covered with a droplet of capture antibody during incubation. The border between droplet and sensor surface (silicon wafer chip) gives under the microscope the impression of a shallow cavity. e SEM micrograph of the sensor surface after MNP (gray spheres) binding

• 

  Fig. 3. Optimization of the detection (n = 3). a The dependence of detector signal (bars) and signal to noise ratio (S/N, lines) on the magnetic field intensity. Hollow squares: S/N of the low concentration reference standard; hollow circles: S/N of the high concentration reference standard. b The influence of the size of magnetic particle on the detector response (bars), and the S/N ratio (line)

• 

  Fig. 4. The standard curves (n = 3) obtained with a low concentration reference standard on GMR sensor I (black line) and a high concentration reference standard on GMR sensor II (red line). Each data point is the average of three repeat measurements, each carried out using a single assay cartridge. The error bars indicate ± 1 σ

• 

  Fig. 5. Method comparison between two assays. Passing-Bablok regression analysis (a) and Bland-Altman analysis of agreement between the GMR and the Roche Cobas e411 assay (b)