Fig. 1: Design, synthesis, and properties of an activatable ratiometric PA probe for detecting neutrophil elastase (NE).

a The synthetic route of NERP. b Response principle of NERP to NE. c Absorption spectra of NERP incubated with NE (2 U/L) or not. d Fluorescence spectra of NERP incubated with NE (2 U/L) or not. e Synthesis scheme for SPNE via the nano-coprecipitation method. f The average hydrodynamic diameters of SPNE in H₂O. g TEM images of SPNE (n  =  3 independent samples). h Scheme for the ratio of Abs₆₉₅/Abs₈₀₀ SPNE when responsive to NE (2U/L) or not. i The absorption spectra of SPNE upon reacting with various concentrations of NE (0–2 U/L). j Quantification of Abs₆₉₅/Abs₈₀₀ ratios in i (n = 3 independent samples). k Fluorescence spectra of SPNE incubated with NE (2 U/L) or not. l Nonlinear regression analysis of cleavage rate V (mg/mL·s) of SPNE as a function of substrate (SPNE) concentration. m Absorption spectra of SPNE incubated with different concentrations of NE (0–15 U/L). n The Abs₆₉₅/Abs₈₀₀ of SPNE after incubation with different concentrations of NE, calculated from m (n = 3 independent samples). o Absorption spectra of SPNE incubated with NE (2 U/L) for different times (0–20 min). p The dynamic response: the Abs₆₉₅/Abs₈₀₀ of SPNE after incubation with NE for different time (0–60 min) (n = 3 independent samples). q The selective response: the Abs₆₉₅/Abs₈₀₀ ratios of SPNE after incubation with different interfering substance (NQO1: NADH quinone oxidoreductase 1, ALP: alkaline phosphatase, GGT: glutamyltransferase, MMP-2: matrix metalloproteinase 2, PEDs: phosphodiesterase) (n = 3 independent samples). Data are presented as the mean ± SD. Statistical significance was determined using a two-tailed Student’s t-test for pairwise comparisons and a one-way ANOVA analysis of variance for multiple groups.