Portable colorimetric absorbance reader with extended dynamic range via time-resolved sensing

Many bioanalytical methods result in the generation of colorimetric readouts, with color intensity providing information of the concentration of the targeted analyte. However, conventional absorbance readers often saturate at high analyte concentrations, requiring sample serial dilutions when the analyte studied can be present in a wide range of concentrations. Here, we present a portable reader based on a single-photon avalanche diode (SPAD) array performing in-pixel time-resolved photon counting to quantify absorbance over an extended dynamic range. The method relies on a lifetime-based measurement principle, in which absorbance is derived from the photon-arrival decay constant (τ) rather than steady-state intensity, enabling accurate quantification under high-flux conditions. We applied this approach to an influenza B nucleoprotein magnetic particle immunoassay. The SPAD-based reader maintained a monotonic response up to 64 ng mL^-1 within the useable dynamic range, compared to signal saturation above 12.5 ng mL^-1 for a commercial photometer, with limits of detection (LoD) and quantification (LoQ) of 0.0034 and 0.0949 ng mL^-1, respectively, while maintaining coefficients of variation (CV) < 10%. A convolutional neural network trained on 16-bin histograms accurately inferred analyte concentration (R² = 0.986), yielding results consistent with the τ-based calibration, thus demonstrating that the same time-resolved photon information can be quantitatively interpreted either through physical modeling or data-driven inference. By extending the useable absorbance range while preserving sensitivity, this approach improves key analytical figures of merit compared to commercial benchtop equipment, paving the way for high-performance portable point-of-care (PoC) solutions.