Highly stable and recoverable humidity sensor using fluorescent quantum dot film

Abstract

Fluorescent sensors are resistant to electromagnetic interference and are electrically insulated, allowing for highly accurate measurements. Quantum dots (QDs) serve as outstanding sensing materials owing to the unique optical properties such as tunable photoluminescence (PL), excellent visible light activity, and high chemical and physical stability. In this paper, we develop an optical humidity sensor based on a QDs nanocomposite film. The film is made of polyvinyl alcohol (PVA), SiO2 microsphere (SM), and QDs through the layer-by-layer self-assembly method. The mechanism of humidity detection is moisture-induced quenching of the QDs fluorescence intensity. The results reveal that our sensor shows a good linear response to relative humidity in the range of 5% to 97%, a fast response-recovery time of 25 s and 20 s, and good repeatability for more than 50 cycles as well as high stability for over 180 days. Possessing the remarkable property, optical humidity sensors are envisaged for great potential applications in environmental monitoring.

Photoluminescent Humidity Sensors Based on Droplet-Enabled Porous Composite Gels

“…(d) Time-dependent PL intensity of the PVA/QDs-APP composite gel when RH changes between 11% and 93%. (e) Comparison of response and recovery time between this work and other typical optical humidity sensors in literature. − ,− (f) Cyclic stability of the PVA/QDs-APP composite gel under alternatively changed RH between 11% and 93%. (g) Photograph (upper left), fluorescence photograph (upper right), and SEM image (lower) of the gel after 100 cycles, with scale bars of 1 cm, 1 cm, and 20 μm, respectively.…”

“…Despite the progress achieved in polymer/QD composite-based humidity sensors, some critical challenges remain. Thus far, the QD ligands explored have mainly focused on water-soluble polar molecules like 3-mercaptopropionic acid (MPA) so as to enable dispersibility of QDs in water-rich solvents and improve the moisture sensitivity of the resultant polymer/QD composite. , However, such functionalized QDs usually suffer from significantly reduced PLQYs due to weak QD-ligand interactions and thus less passivated surfaces defects (such as dangling bonds), , which in turn hamper the humidity sensing efficiency and increase the cost. For example, the PLQY of CdSe/CdS/ZnS QDs modified with a hydrophilic polyoxyethylene octadecylamine (PEGO) ligand decreases from 78% to 47% .…”

“…Notably, the response/recovery time of PVA/QD-APP gel was not only the shortest among the three gels but also outperforms most reported optical humidity sensors, even those of resistive-/capacitive-type in the literature (Figure e and Table S1). − ,− For example, the response/recovery time of a porous silicon/CdTe-MPA composite is 120/120 s (5∼56% RH) . Considering that the gel thickness may be a potential factor affecting the humidity sensing properties, we also investigated the relationship between gel thickness (1 to 20 mm range) and humidity sensitivity/recovery time (Figures S17 and S18).…”

“…12−14 For example, Xia et al developed a PVA/SiO 2 −CdTe QD composite film for a sensitive and stable humidity sensor with a wide sensing range of 5−97% RH. 18 Meng et al reported a polystyrene-based fiber doped with CdSe/ZnS QDs that is flexible and sensitive to moisture with response/recovery times of 30/60 s in a range of 19−51% RH. 17 Despite the progress achieved in polymer/QD compositebased humidity sensors, some critical challenges remain.…”

“…Importantly, their luminescent properties are affected by surface chemical environments, e.g., ions, gases, etc. , and are easily tuned by surface ligands. − Therefore, QD-comprising polymer composites are promising for high-performance flexible humidity sensors and have been receiving increasing research interest. − For example, Xia et al developed a PVA/SiO 2 –CdTe QD composite film for a sensitive and stable humidity sensor with a wide sensing range of 5–97% RH . Meng et al reported a polystyrene-based fiber doped with CdSe/ZnS QDs that is flexible and sensitive to moisture with response/recovery times of 30/60 s in a range of 19–51% RH …”