Skin sensitization is a critical endpoint in cosmetic safety assessment, necessitating reliable animal-free testing alternatives. Current established in chemico assays, such as the Direct Peptide Reactivity Assay and Amino acid Derivative Reactivity Assay, are limited by prolonged 24 h incubation periods and their inability to distinguish between direct electrophilic sensitizers and pro-electrophiles requiring metabolic activation or spontaneous oxidation. This study presents the design, synthesis, and validation of NNDNAC (N,N-dimethyl N-(2-(1-naphthyl)acetyl)-l-cysteine), a novel nucleophilic reactivity probe synthesized via a seven-step pathway. A modified naphthalene structure featuring N,N-dimethylamino substituent enhances nucleophilicity of the cysteine sulfur atom, enabling rapid reactivity assessment within an hour incubation using LC-DAD quantification. Comparative validation studies demonstrated that NNDNAC rapidly identified strong electrophilic sensitizers, achieving 100% and 98% depletion rates for p-benzoquinone and 2-methyl-4-isothiazolin-3-one, respectively, within 1 h. Critically, the NNDNAC assay successfully differentiated pro-electrophiles like p-phenylenediamine and 4-aminophenol, which showed negligible depletion at 1 h but significant depletion after 24 h due to auto-oxidation. Furthermore, NNDNAC classified farnesal as a weak sensitizer, aligning with established KeratinoSens™ and LLNA data. The NNDNAC probe represents a significant advancement in skin sensitization assessment, offering a time-efficient, high-throughput platform that not only accelerates screening processes but also provides crucial mechanistic insights through electrophile/pro-electrophile differentiation, significantly improving animal-free toxicological evaluations.

The generation and stability of foam are critical attributes influencing the perceived efficacy and sensory experience of cleansing products like face cleansers and hair shampoos. This study rigorously investigated the influence of water hardness on the foam characteristics of a face cleanser and hair shampoo through integrated macroscopic, microscopic, and rheological analyses. Hard water consistently induced severe foam destabilization, evidenced by significantly increased foam decay and shortened drainage half-lives. Microstructural analysis revealed pronounced bubble coalescence, manifested as reduced bubble counts and elevated mean bubble areas. Rheologically, hard water compromised foam viscoelasticity, leading to diminished complex moduli (G*), earlier G″/G′ crossovers, and heightened phase angles (δ), signifying a rapid transition to a predominantly viscous, unstable state. Conversely, soft water consistently yielded highly elastic foams with robust G* values, maintained G′ dominance, and low δ, indicative of superior structural integrity and temporal stability. Notably, controlled rate viscosity profiles remained unaffected by water hardness. These findings collectively demonstrate that divalent cations fundamentally undermine foam lamellar film stability, inducing profound structural and mechanical degradation. Concurrently, tribological measurements revealed that the face cleanser consistently exhibited higher coefficients of friction in hard water across varying sliding speeds, whereas the hair shampoo displayed a more complex, speed-dependent frictional profile that was comparatively less sensitive to water hardness. This underscores the critical necessity for formulation chemists to mitigate water hardness effects to ensure consistent product performance and sensory attributes.