Quantitative evaluation of surface hydrophobicity/hydrophilicity.
Considering the relevance between the abundance of oxygens and hydrophilicity, the intensity ratio between the external H-bond-induced yellow emission (λem= 549 nm) and the green emission (λem= 505 nm) of HBT-DPI should be able to quantitatively evaluate the surficial hydrophobicity. As a proof of concept, several PVDF membranes with varying surficial hydrophobicity were prepared via plasma modification from 0 to 180 s. The contact angle (θ ) for each modified PVDF membrane surface was measured within the range of 20-130° (Figure S19 and Table S4). Afterward, the membranes were coated with HBT-DPI as mentioned above to collect the ratios of I 549/I 505(Figure 4A). By plottingI 549/I 505 versus cosθ , the calibration curve between theI 549/I 505 and its corresponding θ was defined by a linear equation,I 549/I 505=3.10cosθ+ 2.83 (Figures 4C and S19, Tables S4 and S6). Owing to the influence of non-uniform surface textures, the measurement of the fluorescence intensity showed variations, resulting in an R2 of 0.9787 for the calibration curve. Despite the slightly low R2, we can still conclude the linear relationship between I 549/I 505 and cosθ , and the HBT-DPI fluorescence intensity ratio ofI 549/I 505 increases along with the increasing cosθ , which represents enhanced hydrophilicity.
On this basis, we further prepared a PVDF membrane with a partially modified surface by covering a porous mask before plasma treatment (Figures 4A and 4B). The purpose is to create unevenly distributed hydrophilic areas on the membrane surface. As expected, after merging the membrane in DCM solution containing HBT-DPI and drying, yellower patterns consistent with the pores on the mask were seen while the covered area, which is less irradiated, maintained green color (Figures 4B and 4D). The observation convincingly demonstrates the unique merits of HBT-DPI in mapping the distribution of hydrophilic and hydrophobic areas on a large-scale surface, which is not feasible with the conventional contact angle method. We also estimated a mean hydrophobicity of the heterogeneous surface by corresponding theI 549/I 505 ratio to a calculated contact angle (Figures 4C and Table S8). For instance, the modified membrane 1 exhibited anI 549/I 505 ratio of 1.29 corresponding to a θ of 126.2°, while the modified membrane 2 with more hydrophilic areas gave anI 549/I 505 ratio of 2.28 corresponding to a θ of 104.6° (Figures 4C and 4D, Table S8). Although the calculated contact angle via HBT-DPI mapping is not representing a specific hydrophobicity of any point on the surface, it can provide information on the overall hydrophobicity of a heterogenous and large-scale surface, which is not able to be estimated before. Taken together, our examples demonstrate the potential ofHBT-DPI as a convenient and useful tool to evaluate the surficial hydrophobicity/hydrophilicity, and its advantages over the conventional method in the aspects of mapping the hydrophobic/hydrophilic areas as well as quantification on a large-scale surface, even the surficial modification is heterogeneous.