The changes in phosphorescence can be clearly observed in the macroscopic photos of the composite film under different states, and the emission intensity and luminescence lifetime of the film with pH=13 show a significant decrease compared to the film with pH=7 (Figure 3c). Actually, phosphorescence behavior of Y-TCPP/DBCZ PVA mixed gel at different pH values were studied in detail (Figure 3d). The composite film exhibits blue fluorescence and yellow-green RTP at pH=6 to pH=12. When the pH rises to 13, the fluorescence of the film turns red, and the RTP intensity significantly decreases, accompanied by a decrease in luminescence lifetime.
Monitoring of Sweat pH
In human bodily fluids, sweat is a weakly acidic liquid (4-7) with a wider pH range compared to other bodily fluids, such as blood. Sweat pH sensor can be used to diagnose metabolic alkalosis, which is a metabolic disease. When the pH value of tissue rises beyond the normal range, it will lead to various health problems, such as arrhythmia, abnormal sensation, neuromuscular irritability, etc., and even Seizure and coma in severe cases. Inspired by the photoluminescence behavior of the pH responsive of composite film, a method for monitoring of sweat pH is proposed here.
The pH of sweat is mainly determined by the concentration of lactic acid. The discussion about the effect of lactic acid on the luminescence of the composite film is necessary. Considering the weak acidity of sweat itself and the luminescence change of gel film under alkaline conditions, the gel film was soaked in Tris solution with pH=13 and then dried it. Four common substances in sweat, lactic acid, glucose, NaCl, and urea, were selected to investigate the response selectivity of the gel film. 50 μL 10 mmol·L-1 solution of selected substance was applied to the film and dried. Compared with the original state, lactic acid treatment showed the most significant fluorescence attenuation, while urea had little effect. The effects of glucose and NaCl on the fluorescence of the composite film were almost invisible (Figure 4a). These results demonstrate the selective response towards lactic acid for the composite film. In order to investigate the effect of lactic acid on the luminescence of the gel films, further research is needed on response time and detection limit. Adding 10 μL lactic acid solution of 10 mmol·L-1 onto the gel film and then recording the fluorescence intensity. It is obviously that the fluorescence intensity remains basically unchanged after 6 minutes (Figure 4b). From the graph of the relationship of lactate concentration and the luminescence intensity of the gel film, the fluorescence intensity decreases significantly after the lactate concentration exceeds 3 mmol, while the phosphorescence intensity increases significantly (Figure 4c). The above results indicate that the composite gel film can be applied to the detection of lactic acid in terms of fluorescent changes.