The paper in Nature Communications is here: http://go.nature.com/2HEKxQc
The process of coloring materials has been around for many decades and is used for a wide variety of applications with the simplest examples including the dying of fabric in order to apply different colors. Recently, colored materials are gaining interest for their applicability in the field of optical sensors with these ‘smart’ sensory materials responding to a small environmental change with a clear and immediate color change. The two most common methods for the fabrication of colored materials is to introduce the dye molecule into the feed-mixture of the fabrication process, so-called dye-doping, or to apply the dye after fabrication as a dying step. Both methods are fairly simple and compatible with a wide variety of dyes or pigments, polymer materials and fabrication processes including hot melt extrusion, injection molding and electrospinning. The main disadvantage of both methods is that the dye is only loosely attached to the material by means of physical interactions in the bulk or on the material surface respectively. This can result in significant leaching of the dye when the material comes into contact with liquids or other materials resulting in decreased sensor sensitivity or output signal and possible toxicological response in biomedical applications. The only way to fully suppress dye-leaching from colored materials is to immobilize the dye on the polymer material through coupling it via a covalent chemical bond to the sample through reactive surfaces or extensive dye modification prior to fabrication. These methods are however highly material specific making these approaches time-consuming and costly and residual unreacted functional groups on the surface could result in adverse effects during application. An alternative method for covalent immobilization that bypasses the necessity of a specific combination of reactive groups is based on the use of surface radicals on the material, thereby expanding the range of applicable materials. The plasma dye coating (PDC) procedure immobilizes a pre-adsorbed layer of reactive dye on the surface through radical addition caused by a short plasma treatment after which residual unreacted dye and potential fragmentation products are removed by washing (see Figure). The non-specific nature of the plasma-generated surface radicals allow for a wide variety of dyes, functionalized with radical sensitive groups, to be combined with any polymeric materials, including paper, plastic bags and Teflon. The presence of the radical sensitive group is crucial for coupling of the dye to the surface at low plasma treatment time, thereby avoiding significant dye degradation. The wide applicability, low consumption of dye, relatively short procedure time and the possibility of continuous PDC using an atmospheric plasma reactor make this procedure economically interesting for various applications ranging from simple coloring of a material to the fabrication of chromic sensor fabrics as demonstrated by preparing a range of pH-sensoric materials.