Ultrafine Capillary-Tube Triboelectric Nanogenerator as Active Sensor for Microliquid Biological and Chemical Sensing

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1 FULL PAPER Biological Sensing Ultrafine Capillary-Tube Triboelectric Nanogenerator as Active Sensor for Microliquid Biological and Chemical Sensing Bao Dong Chen, Wei Tang, Chuan He, Tao Jiang, Liang Xu, Lai Pan Zhu, Guang Qin Gu, Jian Chen, Jia Jia Shao, Jian Jun Luo,* and Zhong Lin Wang* A practical, highly flexible capillary-tube triboelectric nanogenerator (ct-teng) is reported as a microfluidic sensor. The ct-teng is composed of an ultrafine tubular sandwich structure of polytetrafluoroethylene capillary tube, double helix aluminum foil, and silicon rubber hermetic tube. For the first time, microliter sampling (sampling volume, 0.5 µl), nondestructive and highly flexible, is achieved simultaneously for a microliquid sensing device. The self-powered ct-teng is capable of outputting selectable electrical signals (1.1 V, 10 na, 0.9 nc) used for sensing a volume of 0.5 µl microliquid due to Maxwell s displacement current generated with energy converted from microliquid flow. It also achieves both total aerobic count monitoring and electrical conductivity (κ) detection. Moreover, the ct-teng is ultrafine/highly flexible structure and enables its application as a microliter magnitude active sensor for qualitative/quantitative detection. This work provides new opportunities for multifunctional sensing and potential applications in microliquid biological and chemical monitoring/detection technology. 1. Introduction The past short lustrum has witnessed a rapid growth of microliter magnitude sensing devices, with the advent of various revolutionary multifunctional sensing elements ranging from RT-CES technology, [1] planar QCM-based sensor, [2] to nanomechanical resonator. [3] However, these sensors cannot Dr. B. D. Chen, Dr. W. Tang, Dr. C. He, Dr. T. Jiang, Dr. L. Xu, Dr. L. P. Zhu, Dr. G. Q. Gu, Dr. J. Chen, Dr. J. J. Shao, Dr. J. J. Luo, Prof. Z. L. Wang Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing , China luojianjun@binn.cas.cn; zlwang@gatech.edu Dr. B. D. Chen, Dr. W. Tang, Dr. C. He, Dr. T. Jiang, Dr. L. Xu, Dr. L. P. Zhu, Dr. G. Q. Gu, Dr. J. Chen, Dr. J. J. Shao, Prof. Z. L. Wang CAS Center for Excellence in Nanoscience National Center for Nanoscience and Technology (NCNST) Beijing , China Prof. Z. L. Wang School of Material Science and Engineering Georgia Institute of Technology Atlanta, GA 30332, USA DOI: /admt satisfy the requirements imposed by the rapid growth of microliter magnitude sensing technology. Due to the inherent mechanism and its material characteristics, it is hard for some conventional electronic sensors to simultaneously achieve microliter sampling, nondestructive and highly flexible, such as piezoelectric, phase demodulation, and electromagnetic effects. A recently discovered triboelectric nanogenerator (TENG) based on triboelectrification and electrostatic induction is flexible and has potential for high sensitivity. [4 11] The TENG has been extensively investigated to effectively harvest arbitrary mechanical energy from ambient environment. [12 18] The TENG is not only a new energy harvesting technology, but also a sensing technology for the new era the era of Internet of Things. Recently, various types of TENGs have been developed for sensing vibration, pressure, temperature, displacement, and solution chemistry. [19 27] Here, we reported a practical, highly flexible capillarytube triboelectric nanogenerator (ct-teng) composed of an ultrafine tubular sandwich structure that enables biological and chemical monitoring/detection of microliquid. This soft ct-teng uses the inner wall of polytetrafluoroethylene (PTFE) capillary tube as an electrification layer, double helix aluminum as the electrode, and the silicon rubber heatshrink tube as the hermetic package. Different from previously reported, the ct-teng enables microliter sampling, nondestructive and highly flexible, achieved simultaneously for a microliquid sensing. The ct-teng is capable of outputting selectable electrical signals (1.1 V, 10 na, 0.9 nc) used for sensing only a volume of 0.5 µl microliquid; meanwhile, it achieves both total aerobic count (TAC) monitoring and electrical conductivity (κ) detection, entirely attributable to the high sensitivity and the electrical conductivity of microliquid. In view of this characteristic, ct-teng as a microliter magnitude active sensor, our work provides new opportunities for multifunctional sensing and potential applications in microliquid biological and chemical monitoring/detection technology (1 of 10)

2 2. Results and Discussion 2.1. The Structure and Fabrication Process of the ct-teng The structure and fabrication process of the ct-teng is schematically illustrated in Figure 1a, the basic unit consists of a capillary-tube structure TENG inside and a hermetic package outside. Figure 1b shows scanning electron microscopy (SEM) images of the inner wall of the PTFE capillary tube, the PTFE molecular sizes were about µm and homogeneous, and the triboelectric layer of the ct-teng has a thickness of about 0.5 mm. Figure 1c(I) shows optical picture of the ct-teng (internal diameter of 0.5 mm, outside diameter of 1.0 mm, and length of 70 mm). The flexible ability of the ct-teng was characterized, as shown in Figure 1c(II V). The structural flexible/resilient ability of the ct-teng assures its multi functional sensing property. A combo photograph of the ct-teng-detecting system has been shown in Figure 1d, including a ct-teng, an injection syringe, and connector, and its four steps working process is presented in Figure 1e. In this detecting system, the injection syringe as a pneumatic power source, controlling microliquid flowing in and out, and microliquid passing the ct- TENG will have streamline flow Working Mechanism of the ct-teng The working mechanism of the ct-teng is based on the triboelectricity generated from the contact electrification process with microliquid/ptfe capillary tube. A key approach for the biological and chemical sensing of ct-teng is to employ an inner wall of PTFE capillary tube as the triboelectricity layer, the spiral aluminum foil serves as the electrode connected with the external load, and the silicon rubber heat-shrink tube as the hermetic package. The ct-teng s counter parts are microliquid, which are also utilized to contact with the inner Figure 1. The structure and fabrication process of the capillary-tube triboelectric nanogenerator (ct-teng). a) Schematic diagram showing the structure of the ct-teng (internal diameter of 0.5 mm, outside diameter of 1.0 mm, and length of 70 mm). b) Top-view SEM images of the PTFE capillary tube. c) Digital images showing the high flexibility of the ct-teng. d) Digital images of the ct-teng. e) Four steps working process of the ct-teng (2 of 10)

3 Figure 2. Working mechanism and the electrical output performance of the ct-teng. a) Working mechanism of the ct-teng and b) cross section. c) Transfer charge quantity (Q sc ), d) short circuit currents (I sc ), and e) open circuit voltage (V oc ) of the ct-teng. f) Stability and durability testing of the ct-teng. wall of PTFE capillary tube. Design with fully sealed structure can effectively deal with the humidity or wet environment. The operation of the ct-teng can be explained as single-electrode model, which consists of four typical states as shown in Figure 2a (lateral view) and Figure 2b (sectional view). Previous studies have shown that when a microliquid droplet flows through an insulating tube, triboelectricity is generated and contributes to the charged surface of water drop and the polymer surface. [28 32] Due to different surface electron affinities, the charges are transferred from the water drop surface to the polymer film surface, leaving positive charges on the water drop surface and negative charges on the polymer film surface. [33 35] The microliquid and PTFE capillary-tube are supposed to be separate (state I in Figure 2a,b), there is no current flow or electrical potentials. When the microliquid contacts and slides on the inner surface of the PTFE capillarytube (state II IV in Figure 2a,b), in order to maintain electrical neutrality, it will cause equivalent positively and negatively charges on the contact surface. [36 38] As the microliquid leaves the PTFE capillary tube, a negative electric potential difference is established between the double helix aluminum electrode and ground. In the short circuit current case, electrons are transferred from the aluminum electrode to ground (state IV) and reach equilibrium. This process produces an instantaneous negative current in the short-circuit condition as driven by the generated potential gradient. As a result, this procedure forms (3 of 10)

4 the fundamental processes of converting mechanical energy into electricity. Based on the converting process, the generated current can essentially be described by polarization charge of the corresponding Maxwell s displacement current as proposed by Wang et al. [5] J D D E PS = ε t = t + t where J D is the Maxwell's displacement current, D is the electric displacement field, E is the electric field, P S is the polarization field, and ε is the permittivity of the dielectrics. The first term is inducted by the varying electric field, and the second term is the current by the polarization field of surface electrostatic charges. The electrostatic filed built by the triboelectric charges drives electrons to flow through the external load, resulting in an accumulation of free electrons in the metal electrode, σ Ι (z, t), which is a function of the gap distance z(t) between the two dielectrics. Thus, the Maxwell's displacement current density is expressed according to the equation Dz σ I( zt, ) dz d1ε0/ ε1 d2ε0/ ε2 JD = = + = σ c 2 t t dt [ d1ε0/ ε1+ d2ε0/ ε2+ z] dσ c z + d t d ε / ε + d ε / ε + z where two dielectrics with permittivity of ε 1 and ε 2 and thicknesses d 1 and d 2, respectively, This equation means that the displacement current density is proportional to the charge density on the dielectric surface and the speed at which the two dielectrics are being separated or contacted. In this entire cycle, this mode works in a way that relies on the charge exchange between ground and metal electrodes, [39,40] and it has also been proved most useful for utilizing the energy from a moving object without attaching an electric connection. This is the electrical output characteristics of the ct-teng. To demonstrate future application, we used a 10 ml injection syringe to pneumatically control the flow of microliquid, and adopt quantifiable connector to control the volume of microliquid. In experiments, deionized water was chosen as the microliquid source and the volume of 100 µl connector. Figure 2c,d shows the electrical output of ct-teng. Figure 2c shows a peak transfer charge quantity at short circuit case (Q sc ) of 4 nc, the output short circuit current (I sc ) reaches a value of 130 na (Figure 2d), the open circuit voltage (V oc ) of 10 V (Figure 2e) in each cycle. The long-term stability/reliability of ct-teng device was examined, as exhibited in Figure 2f. It was found that there was no decrease of its electrical output response, and can remain stable, consistent for over 1000 cycles. (1) (2) observation. We take a constant volume of 100 µl microliquid in each sample, the ct-teng is used as an active sensor for sensing microliquid flow and converting mechanical energy into electricity. The output short-circuit current (I sc ) generated decreases with deionized water, barreled water, and tap water in order (Figure 3a). The generated I sc from barreled water and tap water are 102 and 64 na at a constant volume of 100 µl, which are 78.5% and 49.2% of that for the deionized water; they show the same tendency as that for the open-circuit voltage (V oc ) and transfer charge quantity (Q sc ), as shown in Figure 3b,c. The V oc of barreled water and tap water are 7.6 and 5.1 V, respectively, and the Q sc of barreled water and tap water are 3.1 and 1.6 nc, respectively, which are smaller than those for deionized water. The results indicate that the ct-teng can sensitively sense different kinds of microliquids even at a tiny disparity. This is because the output performance of the ct-teng is affected by the electrolytes in microliquid. Once there are electrolytes in microliquid, triboelectricity will be weakened and decreased to the charged surface of water drop and the polymer surface. Therefore, smaller electrical output than that of deionized water was obtained. On the other hand, we demonstrated that the ct-teng can sense microliter magnitude liquid. The correspond output of ct-teng with different volumes are shown in Figure 3e g. The generated electrical signals of ct-teng lasting decreased as the microliter sampling is varied from 50 to 0.5 µl. Nevertheless, it is notable that the generated I sc, V oc, and Q sc of ct-teng, obtained only at a volume of 0.5 µl deionized water, are 10 na, 0.75 V and 0.5 nc, respectively, as shown in Figure 3e g. This tendency explicitly shows that the output of ct-teng is influenced by the volume of microliquid, which is more consistent with the Maxwell's displacement current density equation (Equation (2)). The results indicate that the ct-teng could sensitively sense the different volumes of microliquid even a tiny change. Finally, we exhibited a newly practical testing in which ten ionic standard solutions are measured by ct-teng at a constant volume of 100 µl, which include Na +, Mg 2+, Zn 2+, Cu 2+, Sn 2+, Al 3+, NH 4 +, Cl, NO 3, and SO 4 2. These solutions are diluted to a mass concentration of 2.5 mg L 1 by deionized water, as shown in Figure 4a c. The generated I sc, V oc, and Q sc of the ct-teng decrease first and then increase with different ionic standard solutions. The electrical conductivities (κ, µs cm 1 ) of ten ionic standard solutions are measured by conductivity meter (Figure 4d). The relationship between the electrical output of ct- TENG and 1/κ is depicted in Figure 4e. With these curves, this is interesting and momentous, the V OC, I SC and Q SC of curves changing tendency to be consistent with 1/k curve at different ionic standard solutions. So, as a potential application, the ct- TENG can be used as active sensors for microliquid biological and chemical monitoring/detection by k attribution The Sensing Characteristics of the ct-teng To demonstrate the characteristics of ct-teng, which can be used for microliquid sensing, the deionized water, barrelled water, tap water are evaluated by the electrical output of ct-teng (Figure 3a c). In the face of the three colorless liquid, make people difficult to distinguish just rests on 2.4. Biological Monitoring by the ct-teng The ct-teng can be used for biological monitoring, for instance, the TAC (cfu ml 1 ) in healthy diet. TAC is an important index for foodstuff and drinking water quality. [41] Take barreled drinking water as an example, the schematic diagram in Figure 5a shows a drinking scenario of barreled water. The relationship (4 of 10)

5 Figure 3. The sensing characteristics of the ct-teng. a) Output I sc, b) V oc, and c) Q sc of the ct-teng for three colorless liquids and digital image of these colorless liquids. d) Digital images of the ct-teng-based different volume connectors and microliter syringes. e) Outputs I sc, f) V oc, and g) Q sc of the ct-teng for different volumes of deionized water from 0.5 to 50 µl. between TAC and drinking time (day) of the barreled water was tested by plate culture colony-counting method, TAC in watergenerated exponential increases with drinking time increases from 0 to 12 d, as shown in Figure 5b. The result shows that the TAC value of over 110 cfu ml 1 has exceeded the standard of water quality at 7 d. The typical macroappearance of TAC is presented with different time (Figure 5c). Influenced by TAC, the total dissolved solids (TDS, ppm) and κ of barreled water were increased nonlinearly with the increase of drinking time by conventional instruments, as shown in Figure 5d,e, respectively. When driven microliter-barreled water flows through a ct-teng with an injector syringe, the electrical output of ct-teng shows the reversing tendency as that of TDS and κ (Figure 5f). This means that output signals of ct-teng can actively response the change of TDS/κ in water from 0 to 60 d, and the I sc, V oc, and Q sc are decreased due to the increase of TAC with the extension of the drinking time. A previous study has verified that these TACs in drinking water are negatively charged, resulting in a partial weakening of the tribocharges on the surface, and the electrical output of ct-teng will decrease. Therefore, the ct- TENG can be used to monitor the total aerobic count of barreled drinking water. Figure 5g shows the rational drinking time of the barreled water by the ct-teng sensing, and hence we provide four drinking intervals according to two sets of output (5 of 10)

6 Figure 4. Different ionic standard solutions testing by the ct-teng. a) Outputs Isc, b) Voc, and c) Qsc of the ct-teng with ten ionic standard solutions (the mass concentration of all is 2.5 mg L 1). d) Electrical conductivity (κ, µs cm 1) characteristics of these ionic standard solutions are measured by a conventional conductivity detector. e) The relationship between 1/κ and output electrical signals of ct-teng with different ionic standard solutions. signals (A set: 191 na, 3.1nC, 6.3 V; B set: 174 na, 2.4nC, 6 V), including (I) the water can be drunk directly (greater A set signals), (II) drinking requires heating (approximate A set signals), (III) drinking not suggested (approximate B set signals), and (IV) absolute undrinkable (significantly under B set signals). Note that the ct-teng is ultrafine, soft, and capable of adapting to the onsite monitoring, microliter sampling, and has no destruction of the sample and therefore has new potential in biological monitoring, such as the safe in diet, health care Chemical Detection by the ct-teng To further explore the possible applications for the developed ct-teng, a practical demonstration in the stimulant scenes of chemical detection, with a disadvantage of conductance sensor that could be remedied, meets the need of a detection volume of at least 2 ml (Figure 6a). Figure 6b shows only 5 µl microliquid that can be used for the κ detection by a ct-teng, which has a volume similar to that of a rice grain. The relationship between κ and concentration (c, mol L 1) of the potassium chloride (KCl) standard solution was shown by conventional conductance sensor (Figure 6c). The outputs Isc, Voc, and Qsc were recorded when driven by the 5 µl KCl solution flow through a ct-teng with the changes of solution c (Figure 6d); these are decreased linearly with the reversing tendency as that of the κ. The reciprocal of output signals 1/Isc, 1/Voc, and 1/Qsc are shows in Figure 6e, and the linear relationship between these signals and the κ is revealed (Figure 6f). According to the linear relationship, the κ can essentially be described by generated electrical signals of ct-teng, and the linear equations are (6 of 10)

7 Figure 5. Biological monitoring by the ct-teng. a) Schematic diagram showing the drinking scenario of the barreled water. b) The relationship between total aerobic count (TAC, cfu ml 1 ) and drinking time (day) of the barreled water was shown by standard plate culture counting method. c) Optical images showing the distribution feature of TAC at different times. d) The relationship between total dissolved solids (TDS, ppm) and drinking time is revealed through TDS detection pen. e) The relationship between κ and drinking time by conventional conductivity detector. f) Output electrical signals of the ct-teng at different drinking time. g) Schematic diagram showing the rational drinking time of the barreled water by a ct-teng monitoring (gives four drinking interval according to two sets output signals of A set (191 na, 3.1 nc, 6.3 V) and B set (174 na, 2.4nC, 6 V), including (I) the water can be drunk directly, (II) drinking require heating, (III) drinking not suggested, and (IV) absolute undrinkable (7 of 10)

8 Figure 6. Chemical detection by the ct-teng. a) Schematic diagram showing the κ detection of the liquid by conventional conductance sensor (minimum volume > 2 ml). b) Digital images showing the 5 µl microliquid can be used to κ detection by a ct-teng. c) The relationship between κ and concentration (c, mol L 1 ) of the potassium chloride (KCl) standard solution was measured by conventional conductance sensor. d) Output electrical signals of the ct-teng with different concentration of the potassium chloride standard solution. e) The reciprocal of the output electrical signals (1/I sc, 1/V oc, and 1/Q sc ) of the ct-teng. f) The relationship between κ and output signals 1/I sc, 1/V oc, and 1/Q sc. g) Schematic diagram showing the κ of the 5 µl sodium chloride (NaCl) liquid is detected by a ct-teng with different concentration and h) partial magnified view κ I = I sc 5 (3) κ Q = (5) Q sc κ V = V (4) oc where κ I, κ V, κ Q are the electrical conductivities of microliquid by ct-teng sensing, and the adjusted R-square values (8 of 10)

9 (correlation coefficient, R) are 0.997, 0.993, and 0.954, respectively. Here, R ranges from 0 to 1, and the greater is the value of R, the closer is the ordered pair points to the line (the regression line). The results show that they are fairly identical and multiple correlation coefficients are bigger. Based on these linear equations, the κ of sodium chloride solution (NaCl, 5 µl) is detected by a ct-teng, with c of NaCl from to 0.01 mol L 1, as shown in Figure 6g,h (partial magnified view). In comparison to detection result (κ 0 ) by a conventional sensor, the κ I, κ V, and κ Q values are in agreement with the κ 0 value, which shows that the κ I result was in better agreement. Therefore, the ct-teng can be used to detect κ of microliquid; more remarkably, the ct-teng has the significant advantage of being highly flexible microliter sampling and simple structure. 3. Conclusions We reported a practical, highly flexible ultrafine ct-teng composed of an ultrafine tubular sandwich structure of PTFE capillary tube, double helix aluminum foil, and silicon rubber hermetic tube for microliter magnitude biological and chemical monitoring/detection, due to Maxwell's displacement current generated from microliquid flow through the ct-teng. This sensing technology of ct-teng has significant edge in terms of high flexibility, low cost, and especially microliter sampling. For the first time, the concept presented here enables an active sensor aiming for biological and chemical monitoring/ detection through merely a few microliter liquid, and even less. The results show that the sensing limit had reached volume of 0.5 µl microliquid, which the stability generated I sc, V oc, and Q sc of 10 na, 0.75 V, and 0.5 nc, respectively. In addition, the ct-teng is capable of providing output electrical signals (I sc, V oc, and Q sc ) corresponding to the electrical conductivity (κ) of microliquid, which has the same tendency as these testing values. As an application, ct-teng can be used as an active sensor for TAC monitoring of the barreled drinking water that the rational drinking time of the barreled water by a ct-teng was proved, we give four drinking intervals according to two sets of output signals of A set (191 na, 3.1nC, 6.3 V) and B set (174 na, 2.4nC, 6 V), including (I) the water can be drunk directly (greater A set signals), (II) drinking requires heating (approximate A set signals), (III) drinking not suggested (approximate B set signals), and (IV) absolute undrinkable (significantly under B set signals). Finally, another potential application can also be developed to finish the electrical conductivity (κ) detection of sodium chloride solution by only using a volume of similar to a rice grain (5 µl), the results show that κ I, κ V, and κ Q values by ct-teng are in agreement with conventional sensor detection (κ 0 ). In summary, our approach of microliquid sensing can be applied to microliter magnitude biological and chemical monitoring/detection. The capability of total aerobic count monitoring and electrical conductivity detection of the ct-teng was demonstrated when applying it as an active sensor, and the potential for the feature of acidproof, alkaliproof, and high-temperature resistance (over 200 C) would enhanced based on the PTFE material's inherent physical and chemical properties. [42 44] Our work provides new opportunities for multifunctional sensing and potential applications in microliquid biological and chemical monitoring/detection technology. In most cases, this may not require to entirely substituting conventional sensor, but realized the perfect fusion of biological, chemical, and TENG sensing. Hence, there is a plenty of room for new potential applications of TENG for the diverse sensing of microliquid, not just for the total aerobic count and electrical conductivity. 4. Experimental Section The PTFE capillary tube (internal diameter of 0.5 mm, outside diameter of 1.0 mm, and length of 70 mm), double helix aluminum electrode (width of 1.0 mm and thickness of 0.1 mm), and silicon rubber heatshrink hermetic package were all commercial. SEM images of the inner wall of PTFE capillary tube's morphology and microstructure are shown in Figure 1b. Subsequently, the inner wall of PTFE capillary tube served as the tribomaterial, and the double helix aluminum foil served as the electrode connected with the external load. The ct-teng's counter parts were liquid electrodes of being detected microliquid, which were also utilized to contact with inner wall of the PTFE capillary tube. The electrical output signals of the ct-tengs were measured by a Keithley voltage preamplifier and Data Acquisition Card. Acknowledgements B.D.C., W.T., and C.H. contributed equally to this work. The authors thank the National Natural Science Foundation of China (Grant Nos , , , ), the National Key R&D Project from Minister of Science and Technology (2016YFA ), Beijing Municipal Science & Technology Commission (Y DF), and the thousands talents program for pioneer researcher and its innovation team, China, for their support. Conflict of Interest The authors declare no conflict of interest. Keywords active sensors, biological and chemical sensing, microliquid, triboelectric nanogenerator Received: August 23, 2017 Revised: September 6, 2017 Published online: November 20, 2017 [1] K. Solly, X. Wang, X. Xu, B. Strulovici, W. 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