Smartphone-based paper LPS sensor: achieving picomolar ultra-sensitive detection

Rapid and portable lipopolysaccharide (LPS) detection is of great clinical significance. In this study, a paper-based (PB) sensor for rapid LPS detection was developed based on a smartphone and ultrafast atom transfer radical polymerization (UATRP). First, the filter paper (FB) was modified with chitosan (CS) and glutaraldehyde (GA). LPS was modified onto FB using an amino aptamer (Apt), and this cis-diol site from LPS was selectively cross-linked with the boronic acid group of the initiator 4-(bromomethyl) phenylboronic acid (BPA). Then, UATRP was performed using carbon quantum dots (CQDs) as the catalyst and methacrolein (MLA) as the functional monomer, to generate aldehyde-rich polymer brushes on filter paper. After the efficient covalent conjugation of abundant aldehyde groups with chlorinated Nile blue (CNB), the paper-based sensor was successfully prepared. RGB values were obtained by a smartphone application (App) for analysis. Under the optimized experimental conditions, the developed sensor demonstrated good performance compared to other PB-based sensors. It achieved a lower detection limit (LOD) of 22.86 pg mL⁻¹ (S/N = 3) and exhibited a wider linear detection range (1.0 × 10² to 1.0 × 10⁸ pg mL⁻¹). Importantly, the sensor exhibited both high selectivity and remarkable operational stability during LPS detection. These results not only validate the sensor's analytical reliability but also provide a strong foundation for clinical applications and broader biomedical uses.