Background and Originality Content
Sweat not only removes heat but also helps excrete other chemicals and metabolites from the body. The lactate concentration of sweat, mainly contributes to sweat pH, depends on metabolism and physical activity, which links to energy metabolism for assessing fatigue. Besides, it can also be a potential biomarker for pressure ischemia, which leads to a shortage of oxygen that is needed for cellular metabolism.[1] Sweat pH beyond the usual range (4-7) may indicate various health issues,[2] thus, monitoring of sweat pH plays important roles in physiological health, nutritional balance, psychological stress, and sports performance.[3-7]Thus far, various non-invasive point-of-care-testing sensing systems based on enzymes have been developed, however, some disadvantages including high cost, poor stability and susceptibilities to environmental influences limit their practical applications.[8] Metal–organic frameworks (MOFs) are potential candidates for non-enzymatic biosensing due to their unique porous structure, high stability and tailored functionalities.[9-13] The integration of functional MOFs with wearable sensors provides important insights into noninvasive technologies to acquire real-time physiological information.[14-18] However, the direct monitoring of sweat pH based on MOF fluorescence sensor has seldom been reported.
Recently, there has been a remarkable increase in the rational design of biosensor based on time-resolved emission characteristics. RTP with persistent luminescence, are more desired, owing to the disadvantages of the bare fluorescence probes including photo-bleaching, fluctuation of the excitation power and so on.[19-21] It was more desired that the fluorescent and phosphorescent dyes exhibited different emission wavelength and opposite stimuli-responsive behaviour, thus the relative changes in fluorescent and phosphorescent intensity might be utilized to monitor the sweet pH. Moreover, the combination of fluorescent and phosphorescent materials to construct dual-mode response provides new opportunities for anti-counterfeiting technology and practical application.[22-26] In this regard, it still remains a challenge to develop MOF-based functional composite materials with efficient dual-mode fluorescent and phosphorescent emission for sweat pH sensing and information encryption.
Herein, we prepared multifunctional composite gel films by the combination of fluorescent MOF and phosphorescent dyes Upon induction of H+, the composite films exhibited decreased fluorescent signal but enhanced RTP, which could be utilized for sweet pH sensing through a dual-mode. In addition, the above multifunctional composite gel films with pH-responsive fluorescence and RTP exhibited a potential in information encryption and anti-counterfeiting application. This work not only provides in-depth understanding of the stimulus-responsive mechanism of MOF-based functional materials but also advances their potential applications toward sweat pH sensing and anti-counterfeiting.
Results and Discussion
Firstly, Y-TCPP MOF was synthesized according to the previous report,[27] as shown in Figure 1a. The successful synthesis of Y-TCPP MOF was proved by scanning electron microscopy-X-ray energy dispersion spectroscopy (SEM-EDS, Figure S1), X-ray photoelectron spectroscopy (XPS, Figure S2) and X-ray diffraction (XRD, Figure S3). According the SEM-EDS and XPS spectra, it is substantiated the existence of C, N, O and Y. The characteristic peaks of 158.2 eV, 284.8 eV, 400.0 eV and 532.0 eV in XPS spectrum are attributed to the Y3d, C1s, N1s and O1s peaks of Y-TCPP MOF. In addition, the prepared Y-TCPP MOF powder showed multiple strong diffraction peaks at 5.82, 7.46 and 8.88, respectively, indicating the formation of MOF ordered structure. Moreover, the disk structure of Y-TCPP MOF can be clearly seen through SEM images, and the size of multiple disk structures stacked together is about 2 µm. Based on the above results, Y-TCPP MOF which is consisted by porphyrin and Y clusters can be considered to be successfully synthesized. Unlike MOF of non-transition metals, Y in this material has more coordination sites to participate in the interaction.
Y-TCPP MOF could emit the red fluorescence under 365 nm irradiation under alkaline condition[27], while have no photoluminescent properties in the pH range of sweat. On the other hand, 7H-dibenzo[c, g]carbazole (DBCZ)[28] emits blue fluorescence and yellow-green RTP under 365 nm excitation.