Superhydrophobic Silica Aerogels Encapsulated Fluorescent

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Superhydrophobic Silica Aerogels Encapsulated Fluorescent Perovskite Quantum Dots for Reversible Sensing of SO2 in a 3D-printed Gas Cell Xu You, Junjie Wu, and Yuwu Chi Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05253 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Analytical Chemistry

Superhydrophobic Silica Aerogels Encapsulated Fluorescent Perovskite Quantum Dots for Reversible Sensing of SO2 in a 3D-printed Gas Cell

Xu You, Junjie Wu, Yuwu Chi*

MOE Key Laboratory for Analytical Science of Food Safety and Biology, Fujian Provincial Key Laboratory of Analysis and Detection for Food Safety, and College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China *E-mail: [email protected] (Yuwu Chi)

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Recently emerging perovskite quantum dots (PQDs) with several excellent optical properties, such as quantum efficiency, narrow band emission, and tunable emission wavelength, have promising applications in solar cells and light emitting diodes. However, relatively rare applications of PQDs can be found in the field of sensing, mainly due to the very easy degradation of PQDs upon exposure to water or ambient humidity. In this work, for the first time CH3NH3PbBr3 PQDs was encapsulated into a superhydrophobic silica aerogels (AGs) to protect PQDs from being degraded by water. The synthesized PQDs@AGs not only maintain the strong fluorescence emission activity of PQDs, but also show excellent stability in the presence of water. Additionally, PQDs@AGs have abundant pores making them very suitable for gas sensing. For improving sensing performances, 3D-printing technology is introduced into gas cell design and fabrication for the first time. Finally, a novel, sensitive, selective and reversible fluorescence sensor for SO2 gas based on the PQDs@AGs functional material and the 3D-printed gas cell has been developed.

INTRODUCTION Atmospheric sulfur dioxide (SO2) pollutant may be from natural sources such as volcanoes,1 and degraded plants,2 and anthropogenic emissions,3 mainly from power plants, industrial processing, domestic and industrial combustion, road and non-road transport, and international shipping. SO2 as one the major air pollutants, has caused negative effects on ecosystems,4 and the human health,5 and even reduced human intelligence according to a most recent study.6 Therefore, it is of great social and economical significance to monitor SO2 gas in a simple, sensitive, selective and rapid 2


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way. In environment, SO2 gas usually can be detected by electrochemical methods,7-10 optical sensing,11-14, and chromatography.15 Among these methods, optical sensing methods, such as IR absorption spectroscopy,13 cavity ring down spectroscopy,14,16 have been most extensively used since gas pollutants can be measured in a direct, rapid and accurate way which is highly desired by environmental monitoring. However, classic optical sensing methods are essentially based on absorption spectrophotometry, suffering from several disadvantages. For examples, the detection sensitivity was usually low due to the small absorption coefficient of gaseous SO2, sometime the size of instrument was quite large for using long path (several to thousand meters) gas cells to enable the detection of low concentration of sample, and relatively complicate sensing cells designing (e.g. using integrated cavity, or coiled hollow optical waveguides) were often necessary.11 Therefore there are still many challenges in developing new optical SO2 gas sensors with high detection sensitivity or small sensing cells in addition to the above mentioned merits (direct, rapid and accurate) of spectrophotometry. In the past years, various new types of organic fluorescence probes and fluorescence-functionalized nanomaterials have shown attractive applications in gas sensing, due to their excellent fluorescence properties and high sensing selectivities.17-23 Therefore, in this work, we tried to develop a novel fluorescence gas sensor for the detection of SO2 gas to meet the challenges in optical gas sensing. First, the 3D-printing technology is adopted in device design and fabrication in order to obtain a miniaturized and optimized fluorescence gas sensing cell. Second, recently emerging perovskite quantum dots (PQDs) with attractive optical properties,24-27 such as excellent quantum efficiency, narrow band emission, and tunable emission 3



wavelength, are used as the fluorescence sensing materials to improve the detection sensitivity and selectivity. However, it is well known that PQDs without any protection are very easily degraded by H2O molecules both in water and air.28 To overcome

the

water-instability of PQDs, superhydrophobic silica aerogels

(AGs),29,30 are used to encapsulate CH3NH3PbBr3 PQDs for the first time and their fluorescence interaction with gases including SO2 are investigated (Scheme 1). The obtained PQDs-functionalized AGs nanocomposites (PQDs@AGs) have shown superhydrophobicity, abundant pores, and very stable and bright fluorescence in air. It has been observed that the fluorescence of the synthesized PQDs@AGs can be strongly, specifically, and reversibly quenched by SO2 gas. On the basis, a new, sensitive, selectively and reproducibly fluorescence gas sensor has been constructed for the detection of SO2 gas.

Scheme 1. The synthesis of PQDs@AGs and the principle for PQDs@AGs based sensor for SO2 gas.

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EXPERIMENTAL SECTION Chemicals and Materials. Methyltrimethoxysilane (MTMS, 98%), methylamine (33% in absolute ethanol), oleylamine (80%-90%), oleic acid (>99.99%), DMF (99.5%), PbBr2 (98%) and HBr (48%) were all purchased from Aladdin, cetyltrimethylammonium bromide (CTAB, 99.8%), hexane (>99.99%) and toluene (99.7%) were both bought from Sinopharm Chemcial Reagent Co., Ltd. (China), ammonium hydroxide solution (NH3, 25%) was obtained from Xilong Scientific Co., Ltd. (China). All above reagents were used as received without further purification. Doubly distilled water was used throughout the experiments. Preparation of superhydrophobic PQDs@AGs. First blank hydrophobic silica AGs were synthesized by hydrolysis and polycondensation of MTMS.30 In a typical procedure, 5 mL MTMS, 0.05 g CTAB and 15 mL distilled water were mixed in a 50 mL beaker and the mixture was continuously and vigorously stirred for 20 min, resulting in a homogeneous mixed solution. Subsequently, 10 μL of ammonium hydroxide was added to the above mixed solution and was kept stirring for a few minutes. At a stirring speed of 1000 rpm·min-1, the mixed solution (water phase) was rapidly poured into hexane (oil phase) with a volume ratio of 0.3 to form a water-in-oil emulsion. After 10 min, the dispersed phase began to form gel and the stirring was stopped. Filtration and washing with alcohol were performed to remove the residual surfactant and chemicals. The washed wet gels were dried in an oven at 80 C for 1 h and then at 150 C for 2 h. Finally, white dried hydrophobic silica AGs were collected after slowly decreasing the oven temperature to 25 C. CH3NH3PbBr3 PQDs were prepared as reported previously.31 First, CH3NH3Br was prepared from methylamine and HBr, and dried before used. Then in a typical synthesis of CH3NH3PbBr3 PQDs, 0.4 mmol PbBr2 , 0.1 mL oleylamine, 1 mL oleoc 5



acid, 0.32 mmol CH3NH3Br and 10 mL dried DMF were mixed and turned into a transparent precursor solution by ultrasonic agitation. And then 0.5 mL of the obtained precursor transparent solution was quickly injected into 5 mL absolute toluene with vigorous stirring. Finally, a bright green CH3NH3PbBr3 PQDs solution was obtained after centrifuging the above precursor-toluene solution at 14500 rpm for 5 min to remove solid precipitation. The preparation of superhydrophobic PQDs@AGs was carried out as follows. The prepared blank hydrophobic silica AGs were ground into fine powder in advance. Then 5 mL of the prepared CH3NH3PbBr3 PQDs solution was added into a bottle containing 0.2 g of the ground hydrophobic silica AGs powder. The suspension was stirred for 3 h, resulting in saturated adsorption of CH3NH3PbBr3 PQDs into the porous hydrophobic silica AGs. The resultant pale yellow solid materials were collected and washed with absolute toluene by centrifugation to remove residual CH3NH3PbBr3 PQDs solution. Finally, the PQDs@AGs were obtained by drying in an oven for 12 h at ambient temperature. The resulting PQDs@AGs were ground once again before further sensing applications. Instrumentation. Scanning electron microscopy (SEM) images were obtained from an American FEI (Nova nano SEM-230). Surface area and porosity of the nanomaterials were measured by an ASAP 2020 Accelerated Surface Area and Porosimetry System (Micromeritics Instrument Corporation). Fluorescence spectra were measured on an F-4600 Fluorescence Spectrophotometer (Hitachi, Japan). The absolute fluorescence quantum yields (FQYs) of studied fluorescence materials were tested by an absolute PL quantum yield spectrometer (Quantaurus-QY C11347-01, Hamamatsu). Fluorescence lifetimes were recorded on an Edinburgh FL-FS920 TCSPC fluorescence spectrophotometer. Other fluorescence data were all recorded 6


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and processed by a USB2000 fiber optical spectrophotometer with LS-450 LED light source (Ocean Optics). A double channel microinfusion pump (WZS-50F6, Smiths Medical Instrument Co., Ltd.) equipped with 50 mL syringes was used to inject gas samples. A 3D-printer (A8, Shenzhen Aurora Technology Co., Ltd, China) was used to fabricate fluorescence gas sensing cells. Designing and fabrication of fluorescence gas sensing cell. Conventionally, fluorescence gas sensing cell used a small quartz tube to load sensing materials in the middle and put blocking materials (such as cotton) at the two end.19 This type of gas cell configuration was simple but suffered from several obvious disadvantages: (1) It was difficult to adjust light paths to obtain ideal situations both for excitation and emission light beams since the cylindrical surface of quartz tube, mainly leading to bad measurement reproducibility and somewhat decreasing sensing sensitivity; (2) The scattering effect of cylindrical glass tube greatly decreased not only the excitation efficiency of fluorescence materials but also the collection validity of emission light, which significantly decreased the sensing sensitivity; (3) large amount of sensing materials were consumed while most of them did not give fluorescence emission since excitation or emission light was difficult to pass through the deeper layer of cylindrical solid fluorescence materials in the glass tube. Additionally, the none-fluorescent sensing material section might cause longer sensing response time. In order to overcome these disadvantages, we design a novel fluorescence gas sensing cell and fabricate it by using 3D-printing technology (Figure 1A). The cuboidal sensing cell body with size of 30×30×60 mm (a1) is printed from polylactic acid (PLA) wires by the 3D-printer. In the middle of the cell body, a thin cylindrical sensing chamber (a2) with a diameter of 5 mm and depth of 2 mm is printed with the axis perpendicular to a optical window (a5) for loading the fluorescence sensing 7



Figure 1.

Schematic diagrams of PQDs@AGs-based fluorescence sensing cell of

top view (A) and side view (B) consisted of the sections: (a1) PLA cell body; (a2) sensing chamber; (a3) gas channel; (a4) cotton blocking chambers; (a5) quartz optical window; (a6) Teflon gas tube. The photos of the 3D-printed gas sensing cell under a white light (C) and a 365-UV lamp (D). The Schematic diagram of the constructed gas sensing system including following sections: (a) gas sensing cell; (b) gas injection pump; (c) LED light source; (d) USB Fiber Optical Spectrometer; (e) Y-type optical fiber; (f) optical filter. 8


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materials (e.g. PQDs@AGs). A gas channel (a3) with diameter of 1 mm is printed across the sensing chamber for passing gas through the sensing materials. Two small cuboidal chambers (a4) with size of 4×4×2 mm are printed adjacent to the sensing chamber for loading absorbent cotton in order to prevent the sensing materials from being blown away by the gas stream. Finally, after loading the sensing materials and blocking cotton, a rectangular quartz optical window (a5) with the size of 25×12.5× 1 mm is sealed with epoxy on the upsides of the above mentioned cylindrical and cuboidal chambers, resulting in an flow-through fluorescence gas sensing cell (Figure 1C and 1D). To guarantee the air-tight of cell body, “100% filling” mode was applied during printing the cell body from PLA wires layer by layer, moreover a thin epoxy layer (ca. 50 m in thickness) was covered on the surfaces of the cylindrical channels and cuboidal chambers. It should be noted here that epoxy does not interfere the fluorescence measurement since it has no fluorescence activity under 470-nm LED that is used as the excitation wavelength in the present work. The fabricated fluorescence gas sensing cell has following advantages: (1) The flat sensing surface enables the light paths be easily adjusted and thus may facilitate the setup of sensing device and significantly improve measurement reproducibility; (2) Large area of flat sensing surface is perpendicular to excitation and emission light paths, which significantly improves sensing sensitivity; (3) The thin sensing chamber designing will greatly decrease the consumption of sensing materials and shorten sensing response time. Construction and Operation of Superhydrophobic PQDs@AGs-Based Sensor for SO2. As shown in Figure 1E, the fluorescence PQDs@AGs-based gas sensor for SO2 mainly consists of the novel fluorescence gas sensing cell (section (a) in Figure 1E) as mentioned above, a pump (section (b) in Figure 1E) for injecting gas 9



sample into the gas sensing cell, a 470-nm LED light source (section (c) in Figure 1E) to stimulate the fluorescence emission of PQDs@AGs, a USB fiber optical spectrometer (section (d) in Figure 1E) for recording fluorescence spectra of PQDs@AGs in the sensing cell, and a “Y-type” optical fiber (section (e) in Figure 1E) for transferring excitation light from LED to the fluorescence sensing cell and emission light (signal) therein to the USB fiber optical spectrometer. A 500 nm longwave-pass filter (section (f) in Figure 1E) is placed in front of the USB fiber optical spectrometer to remove the excitation light reflected by the sensing cell back into optical fiber. The photo of the constructed fluorescence gas sensor is shown in Figure S1A. In order to adjust the distance and position of the Y-type fiber common end to the optical window of the gas sensor for obtaining a high sensing sensitivity and reproducibility, a baseboard and an optical fiber pedestal were printed for mounting the gas sensing cell and optical fiber (photos C-E in Figure S1), respectively. The gas sensor is operated as follows: First, pure N2 is injected into the gas sensing cell for at least 20 min to eliminate the residual gas in the sensor. Then 50 mL SO2 gas samples with a series of concentrations ( prepared by diluting concentrated SO2 with pure N2) are injected into the sensing cell with a flow rate of 2.5 mL/min. The fluorescence emission spectra of PQDs@AGs in sensing cell are recorded every two seconds. After each measurement of SO2 sample, pure N2 gas is injected into the sensing cell for recovering the fluorescence of sensing materials. All sensing measurements are carried out under a constant room temperature (e.g. 25 C) and a relatively humidity (e.g. 30%).

RESULTS AND DISCUSSION Characterization of Superhydrophobic AGs and PQDs@AGs. SEM images 10


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P-b-PQDs

Figure 2.

P-g-PQDs P-g-PQDs

P-r-PQDs

(A) SEM image, (B) TEM image and (C) HRTEM images of AGs; (D)

SEM image, (E) TEM image and (F) HRTEM images of PQDs@AGs.

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show both blank AGs and PQDs@AGs consist of sponge-like agglomerates (Figure 2A and 2D). The fact that the morphology of PQDs@AGs is quite similar with that of blank AGs implies that very small PQDs nanoparticles (smaller than the silica nanoparticles in AGs) are loaded on the surfaces of AGs in PQDs@AGs composites. TEM data show that the synthesized blank superhydrophobic silica AGs consist of uniform but amorphous silica nanoparticles (ca. 20 nm in diameter) aggregating into a mesh-like morphology (Figure 2B and 2C). The morphology of PQDs@AGs is somewhat similar with that of blank AGs, however many small black dots (ca. 5 nm nanoparticles) are decorated among the mesh-like matrix (Figure 2E). Under low resolution TEM, the small black particles seem somewhat different from the regular cubic unprotected CH3NH3PbBr3 PQDs (Figure S2A), probably due to partial electron beam block by silica frameworks inAGs at TEM investigation. High resolution TEM (HRTEM) images show that these small nanoparticles have clear lattice structure with an inter-planar distance of 0.29 nm (Figure 2F), the characteristic (200) plane of cubic phase structure of CH3NH3PbBr3 PQDs (Figure S2B). Furthermore, STEM-HADD image (Figure S3A), area-selective energy-dispersive X-ray spectroscopy (EDX) spectrum (Figure S3B) and EDX maps (Figure S3C) of

PQDs@AGs show that the

characteristic elements (Br, Pb) of CH3NH3PbBr3 PQDs are contained in Si, O matrices, and are condensed at those sites where PQDs crystals are found. These TEM results clearly indicate that PQDs have been successfully loaded in AGs. Brunauer-Emmett-Teller (BET) gas sorptometry were used to inspect the porous natures of the superhydrophobic silica AGs and PQDs@AGs. The experimental results (Figure 3A) show that the blank AGs have a large surface area (458.05 m2/g), implying that these silica AGs have abundant nanopores and are suitable for loading functional materials for gas sensing. The PQDs@AGs composites have an obviously 12


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Figure 3. (A) N2 absorption-desorption isotherms measured for AGs and PQDs@AGs. (B) XRD patterns obtained for AGs and PQDs@AGs.

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lower surface area (270.68 m2/g), also indicating that many PQDs have been successfully loaded into the silica aerogels and thus occupy some nanopores in AGs. XRD patterns show that unmodified silica AGs on Si substrate modification have a broad (ranging from 16-40 degrees) diffraction peak centered at 22 degree, which can be assigned to the amorphous SiO2 in AGs, whereas PQDs@AGs have additional diffraction peaks corresponding to CH3NH3PbBr3 PQDs.32-34 The XRD data indicate that PQDs have been successfully modified in AGs. The load ratio of CH3NH3PbBr3 PQDs in AGs was tested to be 1.5 wt% (weight percentage) for the prepared PQDs@AGs by weighting the decreased mass of CH3NH3PbBr3 PQDs in the supernatant or the increased mass of AGs after establishing the adsorption equilibrium. The load ratio of CH3NH3PbBr3 PQDs in AGs is much higher than that of CsPbBr3 PQDs (less than 0.1 wt%), probably due to the difference in particle size. CH3NH3PbBr3 PQDs have obviously smaller size (ca. 5 nm, see Figure S2) than that CsPbBr3 PQDs (ca. 10 nm),35 which enables the former to enter into porous AGs more easily than the latter and thus results in larger load ratio of PQDs. This is why CH3NH3PbBr3 rather than CsPbBr3 PQDs are selected for functionalizing AGs. Under the white light, the blank AGs are white powder and the PQDs@AGs are pale yellow powder (Figure 4A). When exposed to 365 nm UV light, the blank AGs have no obvious fluorescence activity whereas the PQDs@AGs emit very bright green fluorescence (Figure 4B) , which verifying in optics that that PQDs have been successfully loaded into the silica AGs. The absolute fluorescence quantum yield (FQY) of PQDs@AGs is 66.4%, which is slightly smaller than that of PQDs (80.2%). The average PL decay lifetime of PQDs@AGs is 46 ns (curve (b) in Figure S4), which significantly larger than that (19 ns) of PQDs (curve (a) in Figure S4). The increase in PL lifetime suggests that the encapsulation by the hydrophobic AGs may 14


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Figure 4. (A) The photos of PQDs@AGs (left) and AGs (right) under a white light; (B) The fluorescence photos of PQDs@AGs (left) and AGs (right) under a 365-nm UV light; (C) The microscopic image of PQDs@AGs under bright field; (D) The fluorescence microscopic image under UV light; (E) Fluorescence excitation (blue curves) and emission (green curves) spectra of PQDs@AGs (solid lines) and PQDs (dash lines).

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reduce nonradiative channels in PQDs.36 Under microscope, PQDs@AGs powder consists of uniform particles less than 100 m (Figure 4C), and each particle emits bright green light under excitation of UV light (Figure 4D), suggesting that that fluorescent PQDs are uniformly load into silica AGs. Fluorescence spectrum measurements show that PQDs@AGs have a wide excitation wavelength range 300 to 520 nm with the maximum excitation wavelength at 365 nm (the blue solid curve in Figure 4E), which is quite similar with that of PQDs in toluene (the blue dash curve in Figure 4E). PQDs@AGs have a sharp emission peak around 530 nm (the green solid curve in Figure 4E), which is about 30 nm red-shifted from that of PQDs in the toluene (the green dash curve in Figure 4E). The red-shift in emission wavelength might be attributed to the somewhat aggregation of PQDs in AGs matrix during adsorption process. Similar red-shift due to aggregation can be found in previously reported polymer-protected PQDs.37 Unlike the unprotected CH3NH3PbBr3 PQDs that rapidly lose fluorescence activity as soon as they are exposed to the atmosphere, superhydrophobic PQDs@AGs can keep their excellent fluorescence activity in air for long time (more than 3 months). Even if PQDs@AGs are put into bottle containing water, they will float on the water surface and remain fluorescence activity without obvious change (Figure 5A-B). The extreme stability of PQDs@AGs toward water or environmental humidity can be attributed to superhydrophobicity of the AGs, with a contact angel of 160 (Figure 5C). The protection of PQDs by the superhydrophobic AGs in the PQDs@AGs composites prevent PQDs from being degraded by water. Interestingly, upon exposure to different ambient humidity, the fluorescence of PQDs@AGs increases slowly with increasing ambient humidity (Figure S5). The fluorescence enhancement mechanism is unknown at present stage, but we go on investigating the enhancement effect, hoping to reveal the enhancement 16


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Figure 5. The Fluorescence photos of the PQDs@AGs powder floating on the water surface under a 365 nm UV light taken after 0 day (A) and 7 days (B). The photo of a drop of water on AGs surface taken for measuring the contact angle (C).

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mechanism in the future. Additionally, photostability and thermostability were also investigated. The experimental results show that the fluorescence of PQDs@AGs is quite stable to the illumination of 470-nm LED working light source (Figure S6), and has a reversible response to temperature in the observed range of 20-50 C (Figure S7). Above investigations on the optical properties of PQDs@AGs and related influence factors suggest that the temperature and humidity should be kept constant to obtain a stable fluorescence of PQDs@AGs. The fluorescence interaction between PQDs@AGs and SO2 gas. It is evident from above characterizations that the porous, air-stable and highly fluorescent PQDs@AGs are very suitable for sensitive gas sensing. As an important pollution gas, SO2 was selected to interact with fluorescent PQDs@AGs for evaluating the possibility of developing a new type of SO2 sensor. First, the spectrum of PQDs@AGs in sensing cell was recorded after flowing 50 mL pure N2 through the cell (blue dash line in Figure 6). Subsequently, 1000 ppm SO2 were injected into the gas sensing cell and the emission fluorescent spectrum of PQDs@AGs was recorded after reaching an equilibrium (red line in Figure 6). The absolute FQY of PQDs@AGs decreased from 66.4% to 26.6% when PQDs@AGs reacted with 1000 ppm SO2. Apparently, the fluorescence of PQDs@AGs can be significantly quenched by SO2 gas, implying that PQDs@AGs can be used for sensing SO2 gas. It should be noted here that the maximum emission wavelength of PQDs@AGs measured by USB fiber optical spectrophotometer (equipped with the 470-nm LED light source) has a ca. 10 nm blue-shift compared with that obtained by the classic fluorescence spectrophotometer (Figure 4E). The blue-shift has been revealed to result from partial overlay of the spectrum of 470-nm LED light source on the emission spectrum of PQDs@AGs (Figure S8). The blue-shift due to LED does not affect the quantitative 18


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Fluorescence intensity / a.u.

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5000

Blank (N2) 1000 ppm SO2

4000 3000 2000 1000 0

450

500

550

600

650

700

Wavelength / nm Figure 6. The fluorescence spectra of PQDs@AGs exposed to N2 and 1000 ppm SO2.

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analysis of SO2 (detailed data discussion are provided in pages S9-S10 in the Supporting Information). The fluorescence inhibition of PQDs@AGs by SO2 is sensitive and reversible,which can be observed by naked eyes for the strong fluorescence emission of PQDs@AGs (Figure 7A). In the pure SO2 gas stream, the bright green fluorescence of PQDs@AGs can be quickly (within 10 s) quenched (sections a-e in Figure 7A), showing a fast sensing response to SO2. Subsequently exposed to the pure N2 gas stream, PQDs@AGs can essentially recover their quenched fluorescence within a short time (sections f-j Figure 7A), indicating a good quenching reversibility. The reversible fluorescence inhibition of PQDs@AGs by SO2 might be explained by the mechanism proposed in Figure 7B. In the absence of SO2, PQDs can emit green fluorescence by the excitation of blue light. However, in the presence of SO2, the coordination reaction between S atoms in SO2 molecules and Pb atoms at the surfaces of PQDs nanocrystals results in a non-emission energy transfer, i.e. the electrons in the conduction band (CB) of excited-state PQDs are transferred into the LUMO (*) of SO2. The latter (SO2 molecules) lose their obtained energy from PQDs by non-emission transition, leading to the quenching of PQDs fluorescence. Probably due to the reversible chemical reaction between SO2 and PQDs, which is similar with well known reversible reactions between SO2 and other species such as organic dyes, the SO2-PQDs complexes may be reversibly decomposed by pure N2 gas, causing the recovery of the fluorescence of PQDs. In order to verify the proposed fluorescence quenching mechanism, the PL decay curves were compared for PQDs@AGs before and after reaction with SO2 (curves (b) and (c) in Figure S4). The average of PL decay lifetimes of PQDs@AGs and PQDs@AGs/SO2 systems are basically the same (respectively 46 ns and 42 ns), which indicates that a static rather than a dynamic 20


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(A)

(B)

Figure 7. (A) Fluorescence photos taken every 2 s for the PQDs@AGs-based gas sensor initially passing pure SO2 through the cell (a-e) and subsequently passing pure N2 through the cell (f-j).

(B) The proposed mechanism for the quenching of

PQDs@AGs fluorescence by SO2.

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quenching mechanism is involved in the fluorescence quenching of PQDs@AGs by SO2, i.e. SO2-PQDs complexes are produced during the fluorescence quenching process. Apparently, the above mentioned strong, fast and reversible fluorescence reaction between PQDs@AGs and SO2 enable the development of a novel and good fluorescence gas sensor for SO2 based on PQDs@AGs. Performances of fluorescent PQDs@AGs-based gas sensor for SO2. After being constructed (Figure 1), related experimental conditions such as flow rate of gas stream and sampling time were optimized and major analytical performances of the sensor, such as response time, linear response range and detection limit, recyclability and sensing selectivity were assessed. First, the effect of gas flow rate on the sensor response was evaluated and shown in Figure S9. The fluorescence intensity obtained from the gas sensor almost does not changes with gas flow rate in the range of 1.67 to 10 mL/min, when passing pure N2 (i.e. in the absence of SO2) through the gas cell (cure (a) in Figure S9). This indicates that the embedded sensing layer (i.e. PQDs@AGs) is mechanical and optical stable under N2 gas stream. In the case of SO2 gas, the quenched fluorescence recorded from gas sensing cell decreases slightly (ca. 5% decrease) with gas flow rate over 1.67 to 10 mL/min (cure (b) in Figure S9) within the same sampling time. The more fluorescence inhibition observed at higher flow rate may result from that at a given sampling time more SO2 gas molecules (proportional to flow rate) are pumped into the gas sensing cell making the reaction approach more closely to the fluorescence quenching equilibrium. For decreasing the sampling volume while maintaining sensing sensitivity, 2.5 mL/min flow rate of SO2 gas stream was selected for sensing. Subsequently, the fluorescence response of PQDs@AGs-based sensor to the concentration of SO2 was assessed by passing a series of concentrations of SO2, the 22


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results of which are shown in Figure 8. At a given SO2 concentration and the selected flow rate of gas stream (2.5 mL/min), the fluorescence intensity of PQDs@AGs at 533 nm decreases with sample injection time, and reaches a relatively stable value after 20 min. The time dependence of fluorescent response might be due to the large number of nanopores and large inner surface area of PQDs@AGs that both slow down the exchange of the gas samples in the sensing materials. Therefore, 20 min of sample injection time was selected for further sensing investigation and the fluorescence intensity at 20 min was used for quantitative analysis. For a certain injection gas volume (or injection time), the fluorescence intensity decreases with increasing the concentration of SO2 over a wide investigated concentration range (2 to 1000 ppm, Figure 8A). There is a fine linear relationship between the fluorescence intensity and concentration of SO2 in the range of 0 to 10 ppm (Figure 8B) with a typical fluorescence quenching equation (Equation 1). By the calibration curve shown in Figure 8B or Equation 1, the concentrations of SO2 samples can be quantitatively analyzed. The theoretical limit of detection (with 99.7% confidence),38 LOD=3/m (σ: standard deviation given by 12 parallel blank measurements, m: slope of Equation 1) was calculated to be 155 ppb, which is lower than the maximum permissible concentration of SO2 upon short-term exposure, 0.5 mg/m3 (i.e. 175 ppb) as guided by WHO.39 This indicates that the developed new gas sensor is applicable in monitoring SO2 pollution. 𝐼 = 𝐼0 ― 𝐾𝐶𝑆𝑂2 = 5010 ― 19.80 × 𝐶𝑆𝑂2 (𝑟 = 0.9959)

(1)

The comparison in the analytical performances between our proposed PQDs@AGs-based fluorescence SO2 gas sensor and previously reported SO2 gas sensing techniques (Table S1) shows that the LOD of our sensor for SO2 is lower than 23



5000

5000

4000 4900

B

A

3000

4800 5000

2000

r = 0.9959

5000 4000

1000

4700

3000

IFl (a.u.)

4950

IFl (a.u.)

Fluorescence Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4900 4850

0

2000

4600 0

0

5

200

400 600 [SO2]/ppm

10

4800

800 1000

15

20

0

0

5

2

4 6 [SO2]/ppm

10

15

8

10

20

Time / min Figure 8. Dynamical fluorescent responses of the gas sensor to SO2 gas streams with various concentrations: (A) over a wide range: 2 to 1000 ppm, inset: plot of fluorescence intensity at 20 min vs SO2 concentration; (B) in a low concentration range: 2 to 10 ppm, inset: linear calibration curve.

24


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those

sensors

based

on

UV

absorption

spectroscopy,12

amperometry,8

conductometry,10 and quartz crystal microbalance (QCM),40 and may be comparable to that of IR absorption spectroscopy,13 and electrochemiluminescence,9 but higher than that of highly sensitive SO2 detection technology based on cavity ring down spectroscopy.14 The LOD of the PQDs@AGs-based fluorescence SO2 gas sensor might be improved by synthesizing PQDs with smaller particle size and larger surface area to increase chemical reaction sites between PQDs and SO2 and then increase fluorescence quenching sensitivity, and by using foreign 365-nm UV LED light source instead of the 470-nm LED light sources originally packed with the fiber optical spectrophotometer to improve fluorescence quantum yield (see fluorescence excitation spectrum shown in Figure 4E ). The reproducibility of the SO2 gas sensor was evaluated by measuring the intraand inter-assay variation coefficients (CVs). As listed in Table S2, the CVs of intraand inter-assay are respectively 0.0313 to 0.0873% and 0.3909 to 0.6153%, indicating that proposed PQDs@AGs-based SO2 sensor has good reproducibility and thus satisfied detection precision. Then the sensing recyclability of the developed gas sensor was investigated by passing 1000 ppm SO2 gas and pure N2 stream alternatively through sensing cell. The experimental results show that the sensing sensitivity do not obviously change after 10 cycles of sample-blank detection (Figure S3), showing a good sensing recyclability of the PQDs@AGs-based fluorescence gas sensor. Finally, the selectivity of the constructed gas sensor was studied by comparing the fluorescence responses of the gas sensor respectively exposed to eight gases such as SO2, NO2, CO2, NH3, O2, O3, H2S and N2 probably found in polluting air (Figure 9), and further evaluating the effects of co-existing gases (NO2, CO2, NH3, O2, O3 and 25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fluorescence Intensity / a.u.


5000

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Blank

4000 3000 2000 1000 0 N2

SO2 NO2 CO2 NH3

O2

O3

H2S

Figure 9. Investigation on the selectivity of the PQDs@AGs-based sensor. The concentrations of gases were all 1000 ppm.

26


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H2S) on the sensing of SO2 (Figure S10). Experimental results show that all of possible interfering gases (NO2, CO2, NH3, O2, O3 and H2S) do not show obvious fluorescent quenching activity at the relatively high concentration whereas SO2 has a significant fluorescence quenching ability (Figure 9), and their co-existences do not interfere the sensing of SO2 (Figure S10), suggesting that the presently developed gas sensor based on PQDs@AGs has a good selectivity in sensing SO2 gas. The good sesning selectivity for SO2 might result from the chemical recognition between PQDs and SO2 by forming PQDs -SO2 complexes via S-Pb bonding.

CONCLUSIONS A novel fluorescence sensor for SO2 gas have been developed by using 3D-printed gas flow-through cell and PQDs-functionalized AGs as the sensing material. The designed and fabricated fluorescence gas cell has a relative large flat sensing surface perpendicular to excitation and emission light paths, and a thin sensing chamber, which significantly improves sensing sensitivity, reproducibility and shortening response time. The synthesized PQDs@AGs nanocomposite sensing materials have excellent superhydrophobicity and can thus maintain a strong and stable fluorescence emission upon exposure to water. The constructed PQDs@AGs-based fluorescence sensor can be sensitively, selectively, and reversibly used for the detection of atmospheric SO2 gas.

ASSOCIATED CONTENT Supporting Information

27



Picture of the FL aerogel-based sensor for SO2, Figure S1. TEM images of unprotected CH3NH3PbBr3 PQDs, Figure S2. STEM-HAADF image, EDX spectrum and EDX maps of PQDs@AGs, Figure S3. The PL decay curves of PQDs, PQDs@AGs and PQDs@AGs-SO2, Figure S4. Effect of humidity on fluorescence intensity of PQDs@AGs, Figure S5. Photostability of PQDs@AGs, Figure S6. Effect of temperature on PQDs@AGs fluorescence, Figure S7. Effect of 470-nm LED light source on fluorescence emission spectrum, Figure S8. The effect of flow rate of gas sample on the sensing response, Figure S9. Investigation on the fluorescence quenching of the PQDs@AGs-based sensor by SO2 in the presence of interfering gases, Figure S10. T Comparison of analytical performances of various SO2 sensors, Table S1. Reproducibility analysis of the fluorescence sensor for SO2, Table S2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]; Tel/Fax: +86-591-22866137. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was financially supported by National Natural Science Foundation of China (21675027), the Program for Scientific and Technological Innovation Leading 28


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Talents in Fujian Province, and the Program for Changjiang Scholars and Innovative Research Team in University (No.IRT_15R11).

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 34


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Table of Contents (for TOC only)

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Superhydrophobic Silica Aerogels Encapsulated Fluorescent

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