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.