Charge-switching and on-demand assembly of carbon dots in acidic biofilm microenvironment for synergistic low-temperature photothermal/cationic therapy.

Vancomycin-resistant Enterococcus (VRE) biofilms establish a dynamic acidic microenvironment characterized by a pH gradient from the outer layers (∼pH 6.5) to the inner core (∼pH 5.5). Conventional pH-triggered strategies, limited by low activation thresholds (pH ≤ 5.5), fail to respond effectively across this gradient, resulting in incomplete biofilm eradication. Thus, we herein present novel nanocomposites (NPs) with an elevated pH response threshold of 6.5, formed through electrostatic assembly of vancomycin-conjugated carbon dots (CNDs@Van) and lysine-modified carbon dots (CNDs@Lys). The prepared NPs undergo on demand assembly and charge switching in the acidic biofilm microenvironment. At the biofilm periphery (pH ∼6.5), protonation-induced aggregation initiates low-temperature photothermal therapy, achieving > 90 % biofilm clearance in vitro under mild heating (≤45 °C). As NPs migrate into deeper acidic regions (down to pH 5.5), they release cationic CNDs@Lys, which penetrate the biofilm and cause ≈ 99.99 % bacterial mortality. In a murine subcutaneous infection model, NPs treatment under NIR irradiation led to ∼80 % abscess shrinkage within 2 days and a 3.1 log₁₀ reduction in bacterial load. The synergy of cationic membrane disruption and mild phototherapy allows significant reductions in light intensity and exposure time while minimizing off-target thermal damage. This strategy not only extends the pH-activation range to cover broader biofilm microenvironments, but also enables spatiotemporally controlled dual-mode therapy via pH-gradient-driven assembly and charge switching, offering a precise and effective platform against drug-resistant biofilms.