Drying-Induced Salt Deposition Patterns as a Tool for Label-Free Protein Quantification.

This work reports a label-free analytical strategy based on protein-induced modulation of salt crystallization patterns upon drying. This method relies on the consistent observation that protein-containing saline samples produce distinct salt deposition morphologies compared to protein-free controls. The work first demonstrates the concept of this phenomenon and characterizes the structural features of the resulting salt patterns. Then, systematic experiments with different solution compositions, substrates, surface coatings, and protein types confirm the generality of this differential deposition behavior and its dependence on total protein concentration. Two complementary measurement approaches are evaluated: a custom laser-scattering setup for optical attenuation measurements and a digital image analysis method based on pixel intensity distributions. Both strategies enable quantitative protein detection in simple (casein) and complex (human serum) samples, offering good correlations between signal and concentration and detection limits in the range of 2-18 µg·mL for digital image analysis and 162-205 µg·mL for optical attenuation measurements. These findings introduce an appealing paradigm for protein quantification exploiting drying-mediated crystallization phenomena, with potential for simple and label-free bioanalytical assays.