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Functional Nanostructured Materials (including low-D carbon)

In situ assembly of well-dispersed Ag nanoparticles throughout electrospun alginate nanofibers for monitoring human breath - smart fabrics Jun Zhang, Xiao-Xiong Wang, Bin Zhang, Seeram Ramakrishna, Miao Yu, Jian-Wei Ma, and Yun-Ze Long ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01718 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Applied Materials & Interfaces

ABSTRACT:

Alginate nanofibers assembled with sliver nanoparticles throughout the whole nanofiber were fabricated by three steps including electrospinning of Na-alginate nanofibers, ion exchange between the sodium and silver ions, and in situ reduction of silver nanoparticles. The content, distribution and size of the nanoparticles are controllable by tuning reaction conditions. Ag/alginate nanofibers exhibit good humidity sensitivity in a wide humidity range from ambient RH (20%) to 85% RH. Interestingly, these humidity sensors can be attached to a 3M-9001V mask for monitoring human breath during exercise and emotion changes, and this smart exhibits accurate and continuous human-breath tracking no matter how fast or slow as well as how deep or shallow. The obtained frequencies of respiration during normal, running, delight and sadness conditions were 16, 13, 14 and 8 times min-1, respectively. Moreover, the signal waveform obtained under emotion changes is distinguishable, implying its potential applications in lie detection and interrogation. Thanks to this smart mask could accurately capture the rate and depth of the respiration, providing an effective, low-cost and convenient approach for tracking respiration, it was utilized as smart fabrics in avoiding sleep apnea.

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1. INTRODUCTION One-dimensional (1D) nanomaterials are considered as ideal building blocks for constructing sensor devices due to their unique physical and chemical properties related to the reduced dimension.1,2 As for humidity sensors, 1D nanostructures with large specific surface areas can effectively improve the performance of sensors,3 which have attracted considerable attention for their widespread applications ranging from industries, hospitals, environmental monitoring to human healthcare.4–6 Many methods have been developed to fabricate and assemble 1D nanostructures in form of nanowires, nanorods, nanotubes and nanobelts, such as template synthesis, chemical vapor deposition, and so forth.7–9 Among these methods, electrospinning techniques exhibit superior advantages in simple and rapid fabrications of nanofibers from a number of materials, including inorganics, polymers and organic-inorganic hybrid compounds.10–12 To construct nanofibers with humidity responsive properties, resistive humidity sensors usually utilize doping methods that introduce conductive fillers in nanofibers, such as graphene and carbon nanotube.13,14 However, the loading level of conductive fillers is low, restricted by their spinnability,15 which greatly hampers their performance as humidity sensors. Another method is coating a conducting layer onto the face of nanofibers, and the common used conductive layers are metal and polyaniline.16,17 Nevertheless, the conductive layer is easily delaminated with a long-term use due to the weak adhesion to nanofiber surfaces.18 Therefore, it highly desires a method that could fill conductive materials throughout the entire nanfibers with high concentration and tight adhesion, which would benefit humidity


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sensors. On the other hand, smart fabrics are defined as textiles having abilities to react to external physical stimuli (thermal, electrical, and mechanical, etc.) in the form of sensing, response, and communication.19 Electrospun nanofibers are widely used in textiles, and they would real-time monitor sweat, sphygmus and body temperature in human daily life when nanofibers are endowed with sensing properties.20 For this reason, fabricating electrospun nanofibers filled of conductive materials with a high content for humidity sensors have tremendous application prospect in smart fabrics.

Sodium alginate (Na-alginate) nanofibers are good candidates for this purpose, not only because Na-alginate is a natural polymer possessing unique properties of non-toxicity, biocompatibility and easy extraction from brown sea weeds, but also due to its rich carboxyl groups which could capture metallic cations in solutions to achieve the local enrichment of cations in the inner of nanofibers.21–23 On the other hand, Ag ions have excellent bactericidal performance,24 and they can transform to elementary substances via the reduction reaction which have superb electrical conductivity.25,26 Therefore, Na-alginate nanofibers combined with Ag ions could trigger Ag ions dispersed in the entire nanofibers with a high content due to the ionic bond between the carboxyl and Ag ion, and this is also the starting point for the following in situ reduction reaction to obtain Ag nanoparticles throughout nanofibers with a high concentration. However, previous reports on nanofibers combined with Ag nanoparticles are mostly focused on doping pre-synthesized Ag nanoparticles into nanofibers,27–29 and not only the doping concentration is limited but also the surface of nanofibers having no Ag nanoparticles corresponding to a low electric conductivity,



which seriously hinder their sensing properties. To the best of our knowledge, there have no report on electrospun Na-alginate nanofibers exchanging with Ag ions followed by in situ reduction to form Ag nanoparticles throughout the entire nanofibers with a high content as humidity sensors for monitoring human breath.

Herein, we report a method to prepare Ag/alginate nanofibers that utilizes electrospun Na-alginate nanofibers as substrates for in situ reduction of Ag nanoparticles throughout the entire nanofibers. The prepared Ag/alginate nanofibers with a high nanoparticle content, uniform nanoparticle distribution, and controllable nanoparticle size exhibit excellent humidity responsive properties in a broad humidity range from 20% to 85% RH. These Ag/alginate nanofibers are further equipped into a 3M-9001V mask to monitor human breath by sensing the breath humidity during exercise and emotion changes, and this smart mask exhibit an accurate real-time monitoring no matter how fast or slow as well as how deep or shallow. Moreover, thanks to this smart mask could accurately capture the rate and depth of the respiration, providing an effective, low-cost and convenient approach for tracking respiration, we finally use it to continuously monitor the frequency of breath during sleep, and demonstrate its potential application in avoiding sleep apnea.

2. RESULTS AND DISCUSSION 2.1 Morphology and structure of nanofibers Figure 1 illustrates the fabrication process of Ag/alginate nanofibers, including electrospinning methods to prepare Na-alginate nanofibers, ion-exchange processes to


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form Ag-alginate nanofibers, and in situ reduction of Ag+ to form Ag nanoparticles throughout nanofibers (for details, see experimental sections). In the experiments, it should be noted that Na-alginate precursor nanofibers can be dissolved in water, but they become insoluble after immersing into AgNO3 aqueous solutions. This is because Na-alginate possesses carboxyl groups can readily attract Ag+ cations to induce ion exchange. After a complexation fulfills between Na-alginate and Ag+ cations, all the Na-alginate transforms into Ag-alginate and thus these nanofibers can no longer be dissolved in water. The abilities of attracting Ag+ ions enable Na-alginate even in the inner of nanofibers to collect a relative high concentration of Ag+ ions, and thus benefit the following in situ reduction processes to obtain Ag nanoparticles throughout the nanofibers.

Figure 2a shows SEM images of Na-alginate precursor nanofibers. The mean diameter of these precursor nanofibers is about 240 nm, and the surfaces of nanofibers are quite smooth. After immersing these precursor nanofibers into AgNO3 aqueous solution for ion exchange and followed by in situ reduction via dimethylamine borane (DMAB) with different time, the color of nanofiber membranes changed from original white to yellow and then metallic surface gloss [Figure S1, Supporting Information (SI)]. The morphologies of these Ag/alginate nanofibers were characterized by SEM and shown in Figure 2b-d. It can be seen that Ag nanoparticles occur on the surface of nanofibers leading to a rather rough surface compared with that of precursor nanofibers. Furthermore, the size of Ag nanoparticles becomes bigger with prolonging reduction time from 10 to 30 minutes, and the coverage gradually increases with



increasing reduction time. After 30 minutes reaction, Ag nanoparticles have begun to form a continuous film on the surface of nanofibers leading to a little increased diameter of nanofibers. The Ag content of Ag/alginate nanofibers obtained at three different reduction time is calculated to be 3.11, 7.19 and 12.63 wt % by measuring their weight (for details, see Table S1, SI), which is consistent with their TGA results (Figure S2, SI). Considering the uniform distribution and a relative high concentration of Ag nanoparticles in nanofibers, in the following discussion, we chose the Ag/alginate nanofibers with reduction time of 20 minutes as the samples for further characterization.

To further verify Ag nanoparticles assembled throughout Ag/alginate nanofibers, TEM images were further conducted. Figure 3a show TEM images of Ag/alginate nanofibers, indicating Ag nanoparticles are uniformly distributed on the nanofibers, which is consistent with SEM results (Figure 2c). The inset in top is the HRTEM image of this nanofiber, showing a lattice spacing of 2.4 Å which corresponds to the (111) lattice plane of Ag nanoparticles. The inset in bottom is the SEAD pattern of Ag/alginate nanofibers, exhibiting spotty diffraction rings of Ag nanoparticles. These results demonstrate that the nanoparticles assembled on the surface of nanofibers are indeed Ag nanoparticles. After ultrasonic treatments, Figure S3 shows a fracture surface of an Ag/alginate nanofiber, in which the embedded nanoparticles are as-expected Ag nanoparticles verified by the HRTEM image. In addition, the variation trend in strain of these nanofibers with increasing Ag content is different from that of Ag nanoparitcles only on the surface (Figure S4, SI). Therefore, it can be


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concluded that Ag nanoparticles are dispersed throughout electrospun alginate nanofibers not only on the surface but in the inner of nanofibers.

Figure 3b shows XRD patterns of Na-alginate nanofibers and Ag/alginate nanofibers. The Na-alginate nanofibers show major peaks at 2θ values of 14.98 and 22.81°, which are assigned to the crystalline polyguluronate unit G(110) and polymannuronate unit M(200) of alginates. In Ag/alginate nanofibers, diffraction peaks at 2θ values of 38.12, 44.36, 64.44 and 77.36° correspond to (111), (200), (220) and (311) planes of cubic silver crystalline lattice (JCPDS 04-0783). It can be seen that the peak intensity ratio of alginates and Ag decreases with increasing reduction time, indicating an increasing amount of Ag nanoparticles throughout alginate nanofibers with prolonging reaction time, which is consistent with SEM results (Figure 2b-d).

To understand the chemical mechanism of ion exchange and in situ reduction for Na-alginate nanofibers transforming to Ag/alginate nanofibers, FTIR spectra were further measured. As shown in Figure 4a(1), there are a large number of sodium ions in the Na-alginate backbone, which could be easily substituted by other active metal ions. In the excess of AgNO3 solutions, sufficient Ag+ began to exchange with Na+ ions on the carboxyl of Na-alginate shown in Figure 4a(2), and this process can undertake a complete reaction because carboxyl groups can electrostatically interact with silver ions to form a stable complex. Finally, Ag+ ions throughout the nanofibers were reduced by DMAB shown in Figure 4a(3), in which the silver atoms first



aggregate into clusters and then attracted other silver ions to adhere on the surfaces and form larger particles due to their big specific surface area. Figure 4b shows FTIR spectra of Na-alginate nanofibers and Ag/alginate nanofibers. As for the Na-alginate nanofibers, the absorption band at 3278 cm-1 belongs to hydroxyl group, and the absorption peaks located at 1602 and 1411 cm-1 are attributed to asymmetric and symmetric stretching vibrations of carboxyl group, respectively. Interestingly, after ion exchange and in situ reduction processes, peaks at 1602 and 1411 cm-1 shift to 1573 and 1405 cm-1 for Ag/alginate nanofibers, indicating that carboxyl groups can react with sliver ions via the complexation.

2.2 Conductivity of Ag/alginate nanofibers Figure 5a shows the temperature dependence of the resistances of Ag/alginate nanofibers and silver colloids, whose resistances were measured by a standard four-terminal technique. The resistances of silver colloids obtained at warming and cooling processes well overlap in the whole process and increase linearly with increasing of temperature, exhibiting an obvious metal behavior. Different from silver colloids, the resistances of Ag/alginate nanofibers at these two different processes only overlap at temperature below 250 K, and they exhibit significant differences above 275 K. In the cooling process, the resistance decreases with decreasing of temperature from 350 K to 275 K. In the warming process, the resistance decreased slowly from 275 K to 350 K. These two different changing trends imply that the Ag nanoparticles in Ag/alginate nanofibers plays an important role in the variations of resistance under the cooling process and alginate plays a predominant role in the


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change of resistance under warming process. Moreover, the resistances of Ag/alginate nanofibers exhibit a similar change trend between 250 K and 275 K. In order to verify the reliability of this abnormal change, several tests were repeated under the same condition (Figure S5, SI). This change in temperature dependence of resistance has not been reported in previous publications, which may be related to some phase transitions in the composite nanofibers, and further studies are needed to explain it. Figure 5b shows the current-voltage (I-V) characteristics of Ag/alginate nanofibers under different temperature. These I-V results are consistent with the R-T results in Figure 5a, further verifying their special electrical properties under different temperatures.

2.3 Humidity sensor performance Figure 6a illustrates the working principle of our designed Ag/alginate nanofibers humidity sensor. Briefly, free ions in the moisture of human breath could connect Ag nanoparticles dispersed uniformly throughout the nanofiber to build an efficient conductive path for charge transfer. The relative humidity (RH) will increase with human exhale and decrease with human inhale. The content of free ions in the moisture is a key factor for the changes of the conductivity with the change of breath RH. In other words, the conductivity of the designed sensor increases with exhaling and falls with inhaling. To eliminate the influence of the vibration caused by respiration, we test the current response of Ag/alginate nanofibers based pressure sensor at different pressures (Figure S6, SI) and the interference test from blowing (Figure S7, SI). The results demonstrate that vibrations caused by respiration could



not play an important effect on the humidity response.

Figure 6b shows a series of current response of Ag/alginate nanofibers sensor for dynamic switching between ambient relative humidity (RH, 20%) and different RH (35-85%) at 25 °C. Obviously, when the nanofibers sensor exposed to the moist air of 35% RH, the current through the nanofibers increased abruptly and then gradually reached a relatively stable value. When the sensor was switched to ambient RH again, the current promptly decreased and gradually reached to a relative constant. Interestingly, when the sensor exposed to higher moist air of 74% or 85% RH, the increasing rate of current is much bigger than that of 35% or 43% RH, which shows an increasing trend with increasing of RH. However, it is hard to reach a relatively stable value within such a short time, which may be caused by the special mechanism of this humidity sensor. Under low RH conditions, water molecules from the moisture could only attach to Ag nanoparticles on the surface of the nanofibers without further permeation. This water layer is discontinuous which restricts the free transportation of free ions in the water between adjacent water molecules, so that the current through the nanofibers is low and could gradually reach a relatively stable value. Under high RH conditions, water molecules from the moisture rapidly attached to Ag nanoparticles on the surface of the nanofibers and then continually penetrated to form a continuous water layer and connect Ag nanoparticles inside nanofibers (Figure S8). Hydronium ions (H3O+) are produced as charge carriers under the electric field, and then these H3O+ are transformed to protons (H3O+→H2O+H+). The hopping of these protons between adjacent water molecules is easier due to the continuous water layer,


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and thus the increase rate of current is faster and could not reach a relatively stable value within a short time. Its excellent sensitivity for different RH inspired us the potential application in monitoring human breath, and thus whether it can follow the respiratory rate was further measured. Figure 6c shows the current response of Ag/alginate nanofibers sensor at different breathing rates, which was measured by holding the sensor at a distance of 1.5 cm from the nose. It can been seen that this smart mask could exactly follow breath, no matter how fast or slow is the human breathing. Meanwhile, this sensor exhibits a good stability, which is important for practical applications. Figure 6d displays the current response of this sensor corresponding to a fast breathing. The current of the sensor increases rapidly while breathing out and decreases to a certain value during inspiration, which can accurately record breathing cycles. As shown in Figure 6d, the response time and recovery time are 1.4 s and 1.1 s. Although Ag related materials as humidity sensor already have some studies in recent years, the reported response and recovery time is longer than that in this study, such as 3 s and 1.4 s for Ag@mpg-CN;30 5 s and 8 s for Ag-SnO2/SBA-15;31 4 s and 6.5 s for Ag/SnO2;32 100 s and 125 s for Ag/SBA-15;33 1.5 s and 1.5 s for MoS2/Ag;34 6 s and 8s for Ag-WO3/SiO2.35 This rapid response and recovery time enables Ag/alginate nanofibers as humidity sensor to well follow the respiratory rate.

2.4 Smart mask for monitoring human breath Thanks to good antibacterial performance (Figure S9, SI), mechanical strength and flexibility (Figure S10, SI), and above-mentioned excellent sensitivity and



stability, we decided to use this Ag/alginate nanofibers sensor for real-time monitoring breath signal, and manifest its practicality and feasibility. As shown in Figure 7a, the lightweight and flexible Ag/alginate nanofibers sensor was conformally attached onto the exhaling valve gasket of the 3M-9001V mask to form a smart mask. Figure 7b displays the real-time breath signals recorded by the smart mask under normal and running conditions. It can be seen that the current curve is low and stable under normal condition, and becomes high and fluctuant under running conditions. Meanwhile, it can be concluded from the insets in Figure 7b that the respiration frequencies under normal and running conditions are 16 and 13 times min-1, respectively. These results indicate that we need to slow down the breath rate and increase breath depth to get more oxygen while exercising. The fluctuant curve obtained under running condition could be attributed to the body undulate. In addition, we measured the current response of this smart mask under running and normal conditions after three months (Figure S11, SI), and there is no obvious difference in current intensity compared with that measured three months ago, which indicates its good stability and reusability.

The ability to capture emotion changes via smart fabrics is very useful in interactive electronic robot and other potential applications such as lie detections.36 Of course, accurately distinguish emotion changes is rather complicated and it is not an easy task by only using one kind of sensor. Herein, we just attempt to capture emotion changes from the normal emotion condition, which might be useful for further studies. Figure 7c and 7d shows the real-time breath signals recorded by the smart mask under


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delight and sadness conditions, which were induced by a piece of comedy of Mr. Bean and tragedy of Titanic, respectively. Obviously, both the signals under delight and sadness emotion conditions have distinguishable differences from that of normal condition. The frequencies of the breath under delight and sadness conditions both decrease and the values are about 14 and 8 times min-1, respectively. From Figure 7c and 7d, we might infer that irregular laughs are the reason for fluctuant signals under the delight condition; the sadness triggers a higher breath depth and thus lower respiratory rate, resulting in higher relative humidity which brings about a higher current signal.

Importantly, monitoring human breath during sleep has become a simple yet powerful non-invasive technique for diagnosing sleep apnea, which is one of the worst killers easily been undiagnosed.37 Traditional dedicated biomedical approaches always need the complicated, expensive and inconvenient instruments such as Polysomnography (PSG),38 which is the current gold standard for measuring sleep, handicapping wide practical applications in household. Our smart mask could capture the rate and depth of the respiration, providing an effective, low-cost and convenient approach for tracking respiration. Figure 7e and 7f show the real-time breath signals recorded by the smart mask during sleep conditions and switching conditions between hold breath and normal breath, respectively. Obviously, this smart mask could accurately monitor real-time changes of breath during sleep. We adopt hold breath state to simulate human sleep apnea (Figure 7f). The significant difference in current response during normal and hold breath conditions indicates this smart mask could



give an alarm for sleep apnea and avoid danger.

3. CONCLUSIONS In summary, we prepared a lightweight and flexible Ag/alginate nanofiber sensor that can sensitively monitor human breath. The Ag/alginate nanofibers were fabricated by a facile route including electrospinning, ion-exchange and in situ reduction. The content, distribution and size of Ag nanoparticles were controlled by adjusting reaction time. The obtained Ag/alginate nanofiber sensor exhibits good humidity sensitivity in a broad humidity range from ambient RH (20%) to 85% RH. Interestingly, the humidity sensor could be attached to a 3M-9001V mask for monitoring human breath during exercise and emotion changes, and the smart mask accurately follows the human breath no matter how fast or slow as well as how deep or shallow. The obtained frequencies of respiration during normal, running, delight and sadness conditions were 16, 13, 14 and 8 times min-1, respectively. Moreover, this smart mask could accurately capture the rate and depth of the respiration, providing an effective, low-cost and convenient approach for tracking respiration, which has potential application in avoiding sleep apnea.

4. MATERIALS AND METHODS 4.1 Materials. Sodium alginate (Na-alginate, 60 mesh, 450 mpa·s, M:G = 1:1) was purchased from Qingdao Hyzlin Corporation, Triton X-100, dimethyl sulfoxide


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(DMSO), polyethylene oxide (PEO, Mw=1000,000), dimethylamine borane (DMAB) and silver nitrate (AgNO3) were purchased from Sigma Aldrich. All agents were used without further purification.

4.2 Electrospinning. 0.64 g of Na-alginate, 0.4 g of Triton X-100, 0.16 g of PEO, and 2 g of DMSO were dissolved in 36.8 g of deionized water, and stirred at room temperature for 4 hours to obtain a transparent homogeneous precursor solution. The purpose of introducing PEO is to enhance the viscosity of precursor solutions for electrospinning. This precursor solution was transferred into a plastic syringes equipped with a stainless needle tip whose inner diameter is 0.21 mm. The voltage added to the spinneret was 12 kV, the flow rate was set to 0.6 ml h -1, and the distance from the spinneret to collector was 12 cm.

4.3 Ion-exchange and in situ reduction of AgNO3. The as-electrospun Na-alginate nanofibers were immersed into 30 wt% AgNO3 aqueous solution. Then, the obtained nanofibers were drawn out and washed with deionized water. Subsequently, the washed nanofibers were immersed into 0.1 % DMAB solution with different reaction time varying from 10 to 30 minutes. Mild reducing agent of DMAB rather than stronger reducing agent of NaBH4 used here for preventing the crack of nanofibers during the reduction. Finally, the nanofibers were washed and dried in a nitrogen atmosphere for 6 hours to obtain the final Ag/alginate nanofibers.

4.4 Humidity sensing test of Ag/alginate nanofibers. Different RH conditions (35-85%) at 25 °C were achieved by saturated salt solutions. The current through



Ag/alginate nanofiber membranes (0.8 cm × 1.2 cm) was in situ measured by a digital multimeter. The cyclic measurement was performed by dynamically switching RH between ambient RH (20%) and different RH (35-85%), when the current was simultaneously recorded.

4.5 Assemble of humidity sensor and smart mask. First, two copper wires were fixed on the Ag/alginate nanofiber membranes (0.8 cm × 1.2 cm) by using silver paste as electrodes to assemble a humidity sensor. Then, a square hole with 0.6 cm × 0.8 cm was cut out in the center of the exhaling valve gasket in 3M-9001V, and the humidity sensor was attached to the square hole to form a smart mask.

4.6 Characterization. The morphology of the ultrathin fibers was characterized by scanning electron microscopy (SEM, JSM-6700F). The size and distribution of Ag nanoparticles throughout nanofibers were acquired using a transmission electron microscopy (TEM, JEM-2100Plus). The crystallographic features of the Ag/alginate nanofibers were determined by X-ray diffraction (XRD, Bruker D8). The physical property measurement system (PPMS, Quantum Design) was employed to measure the electrical resistances of the Ag/alginate nanofiber membranes (1 mm × 6 mm × 0.1 mm). The ITECH IT6322 controlled digital source meter (source-drain voltage was 1VDC) and KEITHLEY 6487 were used to measure the real-time I-T curves at different pressure and humidity.

ASSOCIATED CONTENT


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Optical images and thermal properties of Ag/alginate nanofiber membranes, TEM images of Ag/alginate nanofibers, stress-strain curves and resistance versus temperature curves of Ag/alginate nanofibers, current response of Ag/alginate nanofibers from pressure and blowing as interference test, I-V curves of Ag/alginate nanofibers under different humidity, antibacterial test of Ag/alginate nanofibers, and current response of the smart mask after three months. (PDF)

AUTHOR INFORMATION Corresponding Authors *Z.Y. L. email: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51673103),

Shandong

Provincial

Natural

Science

Foundation,

China

(ZR2017BA013), China Postdoctoral Science Foundation (2017M612200), and the Postdoctoral Scientific Research Foundation of Qingdao (2016007 and 2016014).



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Figure 1. A schematic diagram illustrates the fabrication process of Ag/alginate nanofibers. (a) Na-alginate nanofibers prepared by electrospinning. (b) Ion-exchange and in situ reduction processes.

Figure 2. SEM images of (a) as-electrospun Na-alginate nanofibers and (b-d) Ag/alginate nanofibers obtained at different reduction time: (b) 10 min, (c) 20 min and (d) 30 min.


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Figure 3. (a) TEM images of Ag/alginate nanofibers. Insets show the high resolution TEM (HRTEM) image and the selected area electron diffraction (SAED) pattern. (b) XRD patterns of (1) Na-alginate nanofibers, and (2-4) Ag/alginate nanofibers treated by different reduction time: (2) 10 min, (3) 20 min, and (4) 30 min.

Figure 4. (a) Chemical constructions of (1) Na-alginate prepared by electrospinning, (2) Ag-alginate synthesized by ion exchange and (3) Ag/alginate obtained by in situ reduction of silver salt. (b) FTIR spectra of Na-alginate nanofibers and Ag/alginate nanofibers.



Figure 5. (a) Temperature dependence of resistance of Ag/alginate nanofibers and silver colloids with a four-terminal technique. (b) I-V curves under different temperature. The inset is a magnified plot of the square area.


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Figure 6. (a) Schematic diagram of our designed humidity sensor. Inset is a single Ag/alginate nanofiber in which the Ag nanoparticles are connected by free ions in the moisture. (b) Current response of this sensor for dynamic switching between ambient relative humidity (RH, 20%) and different RH (35-85%) at 25 °C. (c) Current response of this sensor at different breathing rates. (d) Current response of this sensor corresponding to a fast breathing.



Figure 7. (a) A photograph of the smart mask. Inset shows details of the smart mask. Current response of the smart mask: (b) under normal and running conditions; (c) under delight and normal conditions; (d) under sadness and normal conditions; (e) under sleep condition; (f) under normal breath (NB) and hold breath (HB) conditions. Insets show the magnified images of the selected areas.


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In situ assembly of well-dispersed Ag ... - ACS Publications

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