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.