Kinetic traces recorded at different H2O2 concentrations are shown in Figure . Their particularly steep sigmoidal shape is characteristic of an efficient autocatalytic process, for which the lag phase that precedes the exponential decrease of absorbance is a function of the triggering H2O2 concentration. Indeed, it is observed that the lower the starting H2O2 concentration, the longer the lag phase. It is also important to note the S-shaped kinetic curve obtained in the absence of H2O2 (blank) attests to a nonspecific autocatalysis triggered by undesirable reactions (the latter initiating the production of either H2O2 or Q/QH2 to eventually lead to the exponential and full consumption of NADH). The time-to-threshold values (i.e., t1/2 defined here as the time required to reach 50% of the maximal absorbance change) extracted from these kinetic traces were used to plot, on a semi-logarithmic scale, the H2O2 calibration curve reported in Figure . The fact that this calibration plot shows a quasilinear relationship between t1/2 and log[H2O2] over nearly two order of magnitude (i.e., from 0.5 to 20 µM H2O2) confirms the autocatalytic nature of the reaction. The deviation from linearity observed at the lowest concentrations (with a tendency to merge progressively with the t1/2 value determined in the absence of H2O2) illustrates the limitation imposed by the nonspecific response on the analytical performances, leading here to a LOD of show a shift in the kinetic traces to longer times along with a reduction in the slope of the sigmoid as the pH decreases, a behavior consistent with slower autoxidation and deprotection rates at lower pHs. Unfortunately, no gain in LOD was obtained, suggesting a relatively similar pH dependence of both probe reactivity and stability. To understand why this new probe results in such a fast autocatalysis but also in this unexpectedly large nonspecific response, we have examined the fate of the probe alone in a Tris buffer (pH 8.5) and its reactivity towards H2O2. The rather fast evolution of the UV-vis spectrum of a dilute solution of P2 (50 µM) over 20 minutes (graph A in Figure ) indicates the relatively poor stability of P2 in the aqueous buffer. Moreover, the lack of isobestic point reveals the probe degradation into several byproducts. Their structural identity was determined from analyses by HPLC and comparison of the UV-visible spectra of degraded probe with reference spectra of different byproduct candidates (see Figures , S4 and SI for details). Independently of the naphthol boronic acid derivative generated from fast hydrolysis of the pinacol group, two byproducts were found, i.e. the 1-naphthol and 1,4-NQ (according to HPLC analysis, in a 1:3 ratio in the fully degraded probe solution). The formation of 1-naphthol is the consequence of the slow first-order protodeboronation of P2, a well-established reaction for aromatic boronates in aqueous media, while the formation of 1,4-NQ was attributed to the slow oxidation of C-B bond by dissolved O2 (indeed, under argon, we observed that P2 and its hydrolyzed homologue only degrade into 1-naphthol -see Figure ). Since the mechanism of this slow probe autoxidation is unknown, we have at first assumed it can be assimilated to a bimolecular reaction between P2 and O2, leading to H2O2 and 1,4-NQH2. Even if slow, this probe autoxidation is clearly a major issue because, unlike protodeboronation which leads to the production of a dead-end product, it triggers the catalytic degradation of P2 via the production of 1,4-NQH2 and regeneration of H2O2, which in turns further deprotects P2 until the latter is fully converted (Scheme 2). This undesirable reaction finally ends up in an important nonspecific response in the absence of H2O2, which is of course detrimental to the analytical performances. Owing to the pinacol hydrolysis occurring together with the other two nonspecific competitive reactions, it was tricky to access the individual kinetics of these reactions. For the sake of simplicity, we have assumed that the 3 rate constants characterizing the probe protodeboronation (kp), autoxidation (kn), and H2O2 deprotection (kd) are almost the same either P2 is with or without its pinacol group. To determine these rates constants, we relied on a UV-vis absorbance measurement at 262 nm, a wavelength selective of the 1,4-NQ. The kinetic traces in Figure and S5 were obtained either in the absence or presence of different H2O2 concentrations. Without H2O2, the change in absorbance is characterized by a small induction period followed by a nonlinear increase until it asymptotically reaches a constant value after more than 20 min. This increase in absorbance indirectly measures the autoxidation rate of P2 through the nonspecific catalytic formation of 1,4-NQ, from which we can estimate kn. In the presence of H2O2, the increase in absorbance starts sooner (and without delay) and faster as the H2O2 concentration is higher (Figure ). Furthermore, with the increase of [H2O2], the maximum absorbance converges progressively towards a same maximal value, indicating complete probe transformation into 1,4-NQ. For the lowest and intermediate concentrations, the fact that the absorbance levels off at lower values suggests that a nonnegligible fraction of the probe converts into another compound, not detectable at this wavelength, which we attributed to 1-naphthol. The fast kinetics recorded in the presence of H2O2 demonstrate the fast H2O2-mediated deprotection of P2, clearly much faster than previously reported for the self-immolative probe P1 (the kinetic traces of which are reminded in Figure for comparison). The trade-off is that the nonspecific self-deprotection of P2 is also faster than P1, therefore explaining the unsatisfactory detection limit of H2O2.