Flexible 2D graphene-coupled ZnS:Mn2+ mechanodetectors for heart rate monitoring

Abstract

Materials with pressure-induced luminescence is a type of important energy-storage materials that can release the stored energy in form of light upon an external mechanical stimulus, which can be used for light sources, visualization of stress distribution and mapping of dynamic pressure of the materials. However, using such type of materials as the wearable applications to monitor health problems receives little attention in the previous works. Herein, the mechanoluminescent (ML) materials, i.e., ZnS:Mn2+, with an broad emission band peaking at 586 nm upon excitation at 320 nm, are prepared by using an auto-clave method by addition of Na2S to a solution containing Zinc Oleate and Mn(NO3)2. Since the pure ZnS:Mn2+ can release the ML under external press, here we have coupled it with the 2D graphene to form 2D graphene-coupled ZnS:Mn2+ mechanodetector in order to make the ZnS:Mn2+ having the supercapacity property and keep the heart rate monitoring when there is no external press. The crystal structure, photoluminescence, ML and electrochemical properties of ZnS:Mn2+ with and without 2D graphene are characterized by several techniques, including X-ray diffraction, transmission electron microscopy (TEM), photoluminescence (PL) and pressure-induced ML spectra, galvanostatic charge-discharge and cyclic voltammetry. Our findings reveal that 2D graphene-coupled ZnS:Mn2+ possesses pressure-controlled luminescence. By dispersing highly conductive polyoxometalate (POM) into graphene-coupled ZnS:Mn2+ and then detecting the relative ML intensity and pressure value, we find the ML intensity and pressure value feature a linear relationship within a certain range and can basically come back its original state after several cycles. Due to the introduction of the conductive 2D graphene and POM, the graphene-coupled ZnS:Mn2+ is not only sensitive to the external mechanical stimulus but also can possess the electric conduction due to the supercapacity property. The heart rate monitoring results of the device show great potential for the wearable applications to monitor human health problem.