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Low-power and High-performance Trimethylamine Gas Sensor Based on n-n Heterojunction Microbelts of Perylene Diimide/CdS Peihua Zhu, Yucheng Wang, Pan Ma, Shanshan Li, Fuqing Fan, Kang Cui, Shenguang Ge, Yan Zhang, and Jinghua Yu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04497 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Analytical Chemistry

Low-power and High-performance Trimethylamine Gas Sensor Based on n-n Heterojunction Microbelts of Perylene Diimide/CdS Peihua Zhu,† Yucheng Wang,† Pan Ma,†,§ Shanshan Li,† Fuqing Fan, ‡ Kang Cui,† Shenguang Ge,† Yan Zhang,*,† and Jinghua Yu*,† †School

of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China Academy of Agricultural Sciences, Jinan, 250316, China ‡ Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China §Jinan

*Corresponding authors: Yan Zhang, Jinghua Yu *E-mail: [email protected]. Phone: +86-531-82767040. *E-mail: [email protected]. Phone: +86-531-82767161.

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ABSTRACT In

this

work,

low-power

and

high-performance

gas

sensors

toward

trimethylamine (TMA) are presented for the food quality control in Internet of Things. An amphiphilic perylene diimide derivative [1,6,7,12-tetra-chlorinated perylene-N-(2-hydroxyethyl)-N′-hexylamine-3,4,9,10-tetracarboxylic

bisimide,

TC-PDI] is synthesized and further employed to construct the organic microrods of TC-PDI and organic/inorganic microbelts of TC-PDI/CdS by the phase transfer method. Due to the formation of n-n heterojunctions, the TC-PDI/CdS microbelts exhibit higher conductivity than the TC-PDI microrods itself, which present an efficient gas sensing platform for TMA determination at room operating temperature with high reproducibility and selectivity. Remarkably, the limit of detection, stability and selectivity of TC-PDI/CdS gas sensor is significantly improved, which ascribes to the efficient charge separation of n-n heterojunctions. More importantly, the fabricated gas sensor provides potential application of “on-site” and “on-line” TMA identification in real systems and suggests an efficient way to develop new hybrid n-n heterojunctions for low-power and high-performance gas sensor.


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INTRODUCTION The development of Internet of Things requires the portable gas sensors which could integrate into intelligent devices, such as consumer electronics, home healthcare, food quality controlling and environmental monitoring systems.1,2 Specifically, as an essential component in real-time monitoring of toxic gases, chemiresistive sensors have received a lot of interests for simple operation, low production cost, and miniaturization.3 The successful integration of gas sensors with smart platforms relies on moderate processing and operating temperature conditions. Such devices must be low-power consumption, which means that they could operate at room temperature without any additional heating device.4-6 Among toxic gases, trimethylamine (TMA) is one kind of gaseous pollutant and exhaled component. Meanwhile, TMA could be regarded as an effective indicator for evaluating seafood quality. It will be of great importance for the fish industries if their production chains are capable of inspecting the fish quality by on-line detection of TMA.7-9 Moreover, it is also a symptom of problems in the renal organ system when the TMA concentration is 0.1-0.2 ppm in a person's exhaled breath.10 Therefore, it is essential to give top priority to the ultrasensitive detection of TMA in the food quality control,

environmental

monitoring,

and

medical

diagnosis.

Various

chemiresistive-type gas sensors were explored based on metal-oxide semiconductors, such as MoO3,11 ZnO,12 SnO2,13 Fe2O3.14 However, these kinds of semiconductor metal oxide-based sensors usually required a high operation temperature (200-400 C), which limited their practical applications. Therefore, exploiting novel materials were highly valuable for the fabrication of low-power gas sensor.15-17 For this motivation, perylenete diimide derivatives (PDI), a kind of n-type organic semiconductors, have attracted great attentions in the construction of low-power gas sensors, and applied for the detection of amine and nitro compounds.18,19 And, PDI has been proven to be excellent building blocks for self-assembled materials with highly ordered structure due to the strong π-π interactions between the planar PDI rings together with the assistance of hydrogen bonding and side-chain hydrophobic interactions, which is expected to be conducive to charge transport.20,21 For example, Zang et al. prepared an ultrathin nanoribbons with perylene tetracarboxylic diimide derivatives, endowing the unique features such as large surface area and continuous nanoporosity, which enabled efficiently gas



sensing of nitro-based explosives.22 Wang et al. constructed a gas sensor based on perylene diimide derivative for the sensitive detection of hydrazine vapors.23 Recently, heterojunctions of organic-inorganic compounds combining the merits of organic and inorganic materials were found to be very effective in enhancing the gas sensitivity and selectivity.24,25 Liu et al. reported that the inorganic/organic (ZnS/PTCDA) nanoparticles exhibited highly sensitive and selective for detecting aniline vapor over many volatile organic compounds.26 Over the past few decades, various gas sensors have been fabricated with p-n heterojunctions materials, but very few studies have been reported based on n-n isotype heterojunctions. As an exceptional example, CdS-based nanocomposites have been much studied as sensing materials for detecting gases due to their excellent transport properties and adsorption/desorption abilities.27-29 Furthermore, CdS consisted of either cubic or hexagonal or a mixed phase can be well decorated on the surface of micro-structure even at low synthesis temperature, rendering stable hetero-structures. The stable hetero-structures could facilitate electron transfer at the interfaces with an aligned band gap, which could promote CdS as a promising candidate for fabricating the heterojunctions of gas sensing.30 Herein, a low-power gas sensor based on organic/inorganic n-n heterojunction for the sensitive and rapid detection of TMA gas at parts-per-billion (ppb) concentrations was demonstrated. By using a perylene diimide derivative [1,6,7,12-tetra-chlorinatedperylene-N-(2-hydroxyethyl)-N′-hexylamine-3,4,9,10-tetrac arboxylic bisimide, TC-PDI] as the organic component, the organic/inorganic n-n heterojunction microbelts of TC-PDI/CdS was fabricated. In addition, studies on single-component TC-PDI microrods were performed for comparison. The n-n heterojunction structure of TC-PDI/CdS staggered energy levels configuration was useful for the efficient charge separation and transportation. In the presence of n-n heterojunctions, TC-PDI/CdS gas sensor showed significant improvements in their sensing performances toward low concentrations of TMA at room operating temperature. Especially, this sensor exhibited excellent response, reproducibility and selectivity towards TMA in real systems of fish. EXPERIMENTAL SECTION Preparation of TC-PDI. 2.65 g 1, 6, 7, 12-tetra-chlorinated perylene dianhydride (5.0 mmol) was added into 100 mL toluene, followed by 2-aminoethanol (5.0 mmol)


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and n-hexylamine (5.0 mmol). The reaction mixture was refluxed under N2 for 12 h. Then the toluene solvent was removed under reduced pressure, and the red residue was further purified by silica-gel column chromatography in 45% yield, (Scheme S1). MALDI-TOF MS (m/z) Calcd: for C32H22Cl4N2O5, 656.348 Found: 657.756, Figure S1. 1H NMR (300 MHz): (CDCl3), (ppm): 0.91-0.95 (t, 3H), 1.25-1.29 (q, 6H), 1.35-1.40 (m, 1H), 1.74-1.79 (t, 2H), 4.01-4.06 (q, 2H), 4.22-4.25 (t, 2H), 4.51-4.55 (t, 2H), 8.71-8.73 (d, 4H), Figure S2. Gas Sensor Fabrication. Prior to the construction of gas sensor, the microstructures of TC-PDI were fabricated by the following procedure, Scheme S2.31 The microrods of TC-PDI (0.6 mg/mL) were performed in a chloroform/methanol solution (1:3) in a vial and were left still for about four days. The microbelts of TC-PDI/CdS

(0.6

mg/mL)

were

performed

in

a

binary

solvent

of

chloroform/methanol solution of cadmium chloride (0.08 mg/mL) (1:3) in a vial and were left still for about four days. Then the microstructures were transferred from the cadmium chloride solution into pure methanol solution, and a stream of H2S gas was pumped into the methanol solution for 2 hours, as illustrated in Scheme 1. The results exhibited good reproducibility under the same experimental conditions. Afterwards, the suspension of microstructures was carefully dropped onto ITO interdigitated electrode (IDEs) with glass substrates. When the solvents were evaporated completely, the densely packed microstructures remained and tightly adhered on IDEs, resulting to the successfully fabrication of gas sensors. Scheme 1. Schematic Diagram of TC-PDI/CdS Microstructures and the Gas Sensor Fabrication.



RESULTS AND DISCUSSION Characterization of TC-PDI Microrods and TC-PDI/CdS Microbelts. The SEM and TEM images of aggregates of TC-PDI and TC-PDI/CdS were shown in Figure 1 and Figure S4. TC-PDI assembled into rod-like microstructures with ca. 2 μm width and 100-200 μm length in methanol, which mainly depended on the intermolecular π-π stacking interactions, hydrogen bonding and van der Waals interaction,32 Meanwhile, TC-PDI/CdS system induced the formation of microbelts with uniform size and orientation. The microbelts were ordered over several hundred micrometers in length and approximately 1 μm in width, showing the inorganic material effect on the morphology of self-assembled microstructures.33 The morphology change of TC-PDI/CdS microbelt was due to the induction of Cd2+ during the formation process of aggregates.34 The addition of Cd2+ introduced the formation of Cd-O coordination bonds between the cadmium centre and the hydroxyethyl groups of TC-PDI, which counter-balanced the hydrophobic interactions between side chains. The additional Cd-O coordination bonds, incopration with the TC-PDI intermolecular interactions, lead to the belt-like morphology for TC-PDI/CdS. It must be pointed out that the size- and morphology-controllable microstructures are highly desired for developing microscale electronic devices which often required a series of channel lengths to achieve the optimum gate.

Figure 1. SEM images of TC-PDI microrods (A, B) and TC-PDI/CdS microbelts (C, D).

The electronic absorption spectra of TC-PDI in chloroform, TC-PDI microrods and TC-PDI/CdS microbelts were displayed in Figure 2A, and the corresponding data


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were listed in Table S1. TC-PDI in chloroform exhibited characteristic peaks of the non-aggregated perylene diimide chromophores.35 The absorptions at 519 and 486 nm was ascribed to the 0-0 and 0-1 vibronic band of the electronic S0-S1 transition, respectively, while the absorption at 424 nm was assigned to the S0-S2 transition.36 The electronic absorption spectra of the aggregates formed from TC-PDI in methanol were significantly different from that of the same complex dissolved in chloroform. Pronounced band broadening of absorptions was found as a result of the intermolecular interaction in the self-assembled microstructures. Comparing to the spectra of chloroform solution, the bands of TC-PDI microrods (Figure 2A) were blue-shifted to 511, 470 and 422 nm, which revealed the formation of H-type aggregates during the self-assembly process of TC-PDI in methanol. In addition, the main bands of TC-PDI/CdS microbelts formed in the methanol of CdS system were also blue-shifted to 515, 476 and 418 nm relative to that in chloroform solution, indicating the formation of H-type aggregates. Especially, a new band of TC-PDI/CdS microbelts emerged around 596 nm. Actually, the additional band at longer wavelength was a typical sign of effective - interaction in a co-facial configuration of molecular stacking.37,38 The different self-assembly behaviours of TC-PDI in the different solvent system as revealed by the electronic absorption data indicated the effect of the Cd-OH interaction in the self-assembly process. These Cd-OH interactions counterbalanced the TC-PDI intermolecular interactions and the hydrophobic interactions between side chains, leading to well-organized molecular packing mode. The IR spectra of TC-PDI and its microstructures were compared in the Figure 2B, respectively. In the IR spectra of TC-PDI, the broad and strong band at 3432 cm-1 was clearly due to the asymmetrical O-H stretching vibration of the hydroxyl group.39 The vibration of corresponding microrods was blue shifted to 3437 cm-1, which revealed the presence of hydrogen bonds between the hydroxyl groups of neighboring TC-PDI molecules. However, in the IR spectra of the TC-PDI/CdS microbelts, the O-H stretching vibration took an obvious shift from 3432 cm-1 to 3443 cm-1, which demonstrated the forming Cd-OH coordination bonds between the cadmium centre of TC-PDI and the hydroxyethyl groups in the neighbouring molecule in the TC-PDI/CdS microbelts.



Figure 2. (A) Electronic absorption spectra of TC-PDI solution (black line), TC-PDI microrods (red line) and TC-PDI/CdS microbelts (blue line) dispersed in methanol. (B) IR spectra of TC-PDI (black line), TC-PDI microrods (red line) and TC-PDI/CdS microbelts (blue line). XRD spectra of TC-PDI microrods (C) and TC-PDI/CdS microbelts (E). Schematic diagram of the unit cell for TC-PDI microrods (D) and TC-PDI/CdS microbelts (F).

The internal structures of the TC-PDI microrods and TC-PDI/CdS microbelts were studied by XRD analysis, Figure 2C-F. The XRD diagram of the TC-PDI microrods showed two peaks at 1.70 and 1.07 nm in the low angle range in Figure 2C, which originated from the diffraction of the (100) and (001) planes respectively. The XRD pattern also displayed higher order refractions for the two planes at 0.85, 0.53, and 0.36 nm, which were attributed to the diffractions from the (200), (002) and (003) planes, and revealed the long range molecular ordering nature along these directions. Moreover, a broad peak at 0.42 nm for the TC-PDI microrods implied the liquid-like ordered packing of long polyoxyethylene chains.40 The XRD pattern of microrods presented additional two refractions at 0.31 and 0.23 nm in the wide angle region, which is attributed to the π-π stacking distance of TC-PDI rings between the neighbouring molecules and the distance between hydrogen bonding hydroxyl oxygens in the dimer.41 The XRD diagram of the TC-PDI/CdS microbelts presented two distinct peaks at 2.40 and 1.07 nm ascribed to the (100) and (001) planes respectively, Figure 2E.


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Surprisingly, these two planes offered their higher-order refractions at 1.20 (200), 0.60 (400), 0.40 (600), 0.53 (002), 0.36 nm (003) respectively. The series of higher order refractions of the two basic planes illuminated the advantage of crystal growth along these directions, thereby resulting in ordered microstructures with a belt-like morphology for TC-PDI/CdS. The peaks at 0.31 and 0.22 nm were found in the wide angle region, which ascribed to the π-π distance between perylene rings of neighbouring stacking TC-PDI molecules and the distance between hydrogen bonding hydroxyl oxygens in the dimer, respectively.42 Moreover, the XRD pattern displayed three peaks at 0.33, 0.29 and 0.21 nm ascribed to the (111), (200) and (220) planes of the cubic phase of cadmium blende CdS according to JCPDS file No. 65-8873.43,44 To confirm the contribution of cadmium sulfide in the forming microbelts, energy-dispersive X-ray spectroscopy (EDS) and mapping were used to confirm the composition of the microstructures.45 As shown in Figure S5A and S6 , the elemental signature for C, Cl, N and O were observed for the TC-PDI microrods, which clearly indicated the composition of the microrods from corresponding TC-PDI compound. However, the additional elemental signature for the S and Cd atoms were also found in the EDS spectrum and mapping for the TC-PDI microbelts in the presence of CdS in Figure S5B and S7, which demonstrated that CdS were successfully doped into the microbelts. Electrochemical and Electrical Behaviors. The redox behavior of the TC-PDI exhibited two quasi-reversible reductions and one quasi-reversible one-electron oxidation in the electrochemical window of CH2Cl2 Figure 3A and Table S2. The HOMO and LUMO energy levels can be calculated from the first oxidation and reduction half-wave potential values. According to the formula of EHOMO = −E1/2oxd1 − 4.44 eV and ELUMO = −E1/2red1 − 4.44 eV, the HOMO and LUMO energy levels of TC-PDI were evaluated to be -6.14 and -4.00 eV, respectively.46 The LUMO energy levels of -4.00 eV for TC-PDI matched the work function of ITO electrode (-4.50 eV) very well, which revealed the potential n-type semiconductor nature of TC-PDI. The conduction band (Ec) of CdS was -4.1 eV and valence band (Ev) was -6.5 eV room temperature. Herein, TC-PDI and CdS were well self-assembled by the phase transfer methods and yielded n-n heterojunction. The uniform microstructures of TC-PDI/CdS would be potential candidates for the fabrication of electronic devices. The diluted suspension of microstructures of



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TC-PDI and TC-PDI/CdS was carefully dropped on IDEs, and measured electron conductivities in situ respectively. Figure 3B showed the current-voltage (I-V) characteristics of TC-PDI microrods and TC-PDI/CdS microbelts. According to the previously reported equation,47,48 the electronic conductivities were calculated to be 1.15×10-3 and 4.97×10-3 S·cm-1 for the TC-PDI microrods and TC-PDI/CdS microbelts, respectively. This is indicative of improved conductivity of TC-PDI/CdS microbelts compared with TC-PDI microrods, due to the higher ordered crystalline molecular

arrangement

and

n-n

heterojunctions

structure.49

The

present

microstructures with such a high modulation of conductivity could be useful in a wide range of electronic and sensor devices.

Figure 3. (A) Cyclic voltammogram of TC-PDI. (B) I-V curve of TC-PDI microrods (black line) and TC-PDI/CdS microbelts (red line).

Gas Sensing Properties. As shown in Figure 4A and B, systematically changes of current were observed when the concentrations of TMA decreased from 100 ppm to (20) 1 ppm. Upon turning off the TMA source, current intensity recovered quickly to its initial level, which indicated that TC-PDI and TC-PDI/CdS-based sensors exhibited rapid response and reversibility towards TMA. It was obviously observed that the current response gradually enhanced with the TMA concentration increased in Figure 4C. The obtained current curve exhibited a good linear relationship with the concentration of TMA ranging from 1(20)-60 ppm and 60-100 ppm (Adj. R-square>0.95), respectively. By Vineet Dua’s reported method,50 the theoretical low detection limits (for signal-to-noise ratio of 3) for TC-PDI/CdS sensor were estimated to be 200 ppb, which was lower than or equalled to the TC-PDI sensor and previously reported results (Table 1). The charge carriers accumulation at the n-n heterojunctions of the TC-PDI/CdS sensor is responsible for the enhanced sensitivity in comparison with the TC-PDI sensor.


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Figure 4. Response-recovery curves of TC-PDI sensor (A) and TC-PDI/CdS sensor (B) to different TMA concentrations. Response of TC-PDI sensor (black line) and TC-PDI/CdS sensor (red line) to different concentrations of TMA (C). Reproducibility of TC-PDI sensor (D) and TC-PDI/CdS sensor (E) to 100 ppm of TMA (5 cycles) (inset: long term stability characteristic of the sensors to 100 ppm TMA at different durations). Responses of the TC-PDI/CdS sensor (F) to hairtail storage time at room temperature.

Figure 4A and B showed that the response and recovery time obtained for TC-PDI sensor to different concentrations of TMA gas was 130 and 610 s, respectively. However, the response time of the device to TC-PDI/CdS sensor was calculated to be 325 s, whereas the recovery time was found to be 510 s. Such high sensitivity and rapid response/recovery time make the proposed sensors highly reliable and useful for the detection of TMA. The recyclability of TC-PDI and TC-PDI/CdS microstructures-based sensors was checked by exposing 100 ppm TMA repeatedly after a certain interval of time, as shown in Figure 4D and E. Almost similar enhancement of current intensity was presented after each exposure, which confirmed the viability of the sensor for practical application. Meanwhile, the long-term stability of the TC-PDI and TC-PDI/CdS gas sensors were also evaluated by determining the response to 100 ppm



TMA and storing over 30 days in air at room temperature, and the results are shown in the inset of Figure 4D and E. A slight response fluctuation of the TC-PDI sensor could be observed after storing for 30 days. However, it was apparent that there was no significant change in the response of the TC-PDI/CdS sensor after storing for 30 days. Furthermore, the selectivity is also important of the sensing characteristics. In fact, sensors with good selectivity could detect target gas in the multicomponent gas environment, especially with similar physicochemical properties. Figure S8 depicted the selectivity of the TC-PDI and TC-PDI/CdS microstructures-based sensors to 20 ppm TMA and 100 ppm of various gases, including ethanol, acetone, benzene, ammonia and NO2. It is clear that the TC-PDI and TC-PDI/CdS sensors exhibited the largest responses towards TMA among the tested gases, which ascribed to the strong hydrogen-bonds interactions between hydroxyl groups of TC-PDI and nitrogen of TMA.31 These results indicated that the TC-PDI/CdS sensor had excellent sensitivity, stability and selectivity towards TMA gas at room operating temperature, and fitted for “on-line” detection of TMA. Because the fish were stored in high humidity environment, it is indispensable to analyse the influence of humidity on the sensing response for TMA. It can be seen that the TC-PDI and TC-PDI/CdS sensors showed similar response to the variation in the humidity conditions from 30 % to 100 % RH, Figure S9. When the sensors was used in different humidity, the change of sensing response was very small, which suggested that such sensors can be applied in relative humidity for the TMA detection. Table 1. Comparison of TMA-sensing properties in this work and some reported literatures. Materials LOD Fish Ref. Topt.(℃) TMA Conc. Response (ppm) (ppm) freshness In2O3-SnO2 NFs 280 10 7.11a 1 No 51 a α-Fe2O3/TiO2 250 50 13.9 10 No 52 a [Co(im)2]n 75 2 2.5 2 No 8 a α-Fe2O3 217 50 14.9 0.1 Yes 22 NiGa2O4 25 10 1.6a 10 No 53 a MoO3 nanobelts 240 1 6 No 19 b TC-PDI/CdS 25 10 18% 0.2 Yes This microbelts work Topt=optimal operating temperature. TMA Conc.=TMA gas concentration. a = Ig/Ia, b = (Ig-Ia)/Ia; LOD=limit of detection.


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Enhancement Mechanism of Gas Sensing Behaviors. The operating principle of the TC-PDI/CdS sensors is based on the resistance change when the surface reaction with target gases occurring on the sensing layers. In good agreement with the electrochemical analysis, the TC-PDI showed n-type sensing characteristics. In ambient air, the oxygens were adsorbed on the surface of sensing layers, leading to the formation of the chemisorbed oxygen ions.54 O2 + 2e- → 2O-(ads)

(1)

Once TMA molecules were adsorbed on the surface of sensing layers, the trapped electrons in the chemisorbed oxygens were donated back by the following reaction.14 2N(CH3)3 + 21O-(ads)↔ N2+ 6CO2+9H2O +21e-

(2)

Then, the donated electrons immediately transferred to TC-PDI molecules in TC-PDI/CdS microbelts, and resulted in the increased current with the increase of TMA concentration, as illustrated in Scheme 2A. This illuminated the linear relationship between the response and TMA concentration. Scheme 2. Electron-transfer Mechanism (A) and Proposed Sensing Mechanism (B) of the TC-PDI/CdS Microbelts to TMA.

In the process of adsorbing the TMA molecules on the sensing layer,



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double-hydrogen-bonds were formed between carbonyl oxygen of TC-PDI and amino hydrogen of target, which significantly promoted electron transfer between the TC-PDI and TMA molecule, contributing to the high sensitivity of the fabricated device.55,56 On the other hand, the strong molecular interactions and abundant active sites also lead to strong adsorption towards other gases, which reduced the high selectivity of the fabricated device. In order to eliminate this adverse effect, CdS microbelts were introduced to optimize the adsorptive capability and well balance both the high selectivity and sensitivity of the fabricated device. Furthermore, n-n heterojunction was established between TC-PDI and CdS microbelts. Benefiting from the matched energy level between TC-PDI and CdS (Scheme 2B), based on the electrochemical study of TC-PDI and previously reported results for CdS, electrons could transfer to CdS from TC-PDI until the system gets an equilibrium Fermi level (Ef). This bended the energy band and lead to electron injection barrier between the TC-PDI and CdS.57,58 It is commonly considered that the resistance (R) is exponential to the potential barrier height (qV), as described in eq. (3): R=R0×exp(qV/kT)

(3)

Here R0 is flat band resistance, and T represents temperature.59 From eq. (3), it can be concluded that the slight change of potential barrier may greatly affect the material resistance.60 The amplification effect of n-n heterojunction allowed the improvement of the sensitivity to low concentration TMA, which was beneficial for the application of TC-PDI/CdS sensors in fishery production. Application of the TMA Gas Sensors. Considering the practical application, the TC-PDI and TC-PDI/CdS sensors were exposed to volatiles from Hairtail (about 10 g), respectively. The linear relationship between the storage times of Hairtail and responses of these sensors were presented in Figure 4F and Figure S10. The TC-PDI/CdS sensor showed a continuous and accelerating increase when the Hairtail was stored for 0 and 72 hours. A good linear relationship was obtained between the responses of TC-PDI/CdS sensor and the storage times, as presented in Figure 4F. However, there was no similar linear relationship between responses and storage times for TC-PDI sensor, as displayed in Figure S10. It can be seen that the TC-PDI/CdS microbelts sensor can “on-line” and “on-site” monitor the volatiles of Hairtail more accurately than the TC-PDI microrods sensor, which demonstrated that


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such a TC-PDI/CdS microbelts sensor is more suitable for the instant determination of fish freshness. As shown Table S3, the relative error was in the range of 0-7.6 ％ between the proposed and reference methods, which indicated an acceptable accuracy. Then, the as-prepared TC-PDI/CdS gas sensor could be applied to the determination of TMA in real samples with available results. CONCLUSIONS In conclusion, a new organic/inorganic n-n heterojunction of TC-PDI/CdS microbelts was prepared by the phase transfer method for the sensitive detection of TMA. In addition, the single-component TC-PDI microrods were performed for comparative studies. These gas sensors showed a liner relationship between response and TMA concentration. In particularly, the TC-PDI/CdS microbelts exhibited more excellent sensitivity, stability and selectivity towards TMA than the TC-PDI microrods. The results elucidated the synergistic effect between organic and inorganic materials and demonstrated that the n-n heterojunctions was a key aspect for the enhanced sensing properties of the TMA gas sensors. Meanwhile, this study provided a rapid, low-powered and ultrasensitive analysis for the volatiles of fish, which suggested that the proposed sensors enabled the “on-line” and “on-site” monitoring for food quality control. Furthermore, the organic/inorganic n-n heterojunction has been used to enhance the performance of gas sensor for the first time. It is believed that this work offers new insights into the formation of low-power and high-performance gas sensor and may be instructive for the future design of flexible sensing platforms employed in Internet of Things.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional experimental details, results, and discussion (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Phone: +86-531-82767040. *E-mail: [email protected]. Phone: +86-531-82767161. ORCID



Peihua Zhu: 0000-0002-6874-9013 Kang Cui: 0000-0002-4947-4448 Shenguang Ge: 0000-0002-0537-6491 Yan Zhang: 0000-0002-1936-4619 Jinghua Yu: 0000-0001-5043-0322 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors are thankful for support from the National Natural Science Foundation of China (51872121 and 21874055), the Taishan Scholars Program (ts201712048), the Shandong Provincial Natural Science Foundation (ZR2017MB058), the National Postdoctoral Program for Innovative Talents (BX20180129), and China Postdoctoral Science Foundation (2018M640608). REFERENCES (1) Kim, Y. H.; Sang, J. K.; Kim, Y. J.; Shim, Y. S.; Kim, S. Y.; Hong, B. H.; Jang, H. W. Self-Activated Transparent All Graphene Gas Sensor with Endurance to Humidity and Mechanical Bending. ACS Nano 2015, 9, 10453-10460. (2) Han, S. T.; Peng, H.; Sun, Q.; Venkatesh, S.; Chung, K. S.; Lau, S. C.; Zhou, Y.; Roy, V. A. L. An Overview of the Development of Flexible Sensors. Adv. Mater. 2017, 29, 1700375-1700397. (3) Wu, J.; Li, Z.; Xie, X.; Tao, K.; Liu, C.; Khor, K. A.; Miao, J.; Norford, L. K. 3D Superhydrophobic Reduced Graphene Oxide for Activated NO2 Sensing with Enhanced Immunity to Humidity. J. Mater. Chem. A 2017, 6, 478-488. (4) Kauffman, D. R.; Star, A. Carbon Nanotube Gas and Vapor Sensors. Angew. Chem. Int. Ed. 2008, 47, 6550-6570. (5) Chen, K.; Gao, W.; Emaminejad, S.; Kiriya, D.; Ota, H.; Nyein, H. Y. Y.; Takei, K.; Javey, A. Printed Carbon Nanotube Electronics and Sensor Systems. Adv. Mater. 2016, 28, 4397-4414. (6) Duy, L. T.; Trung, T. Q.; Dang, V. Q.; Hwang, B. U.; Siddiqui, S.; Son, I. Y.; Yoon, S. K.; Chung, D. J.; Lee, N. E. Flexible Transparent Reduced Graphene Oxide Sensor Coupled with Organic Dye Molecules for Rapid Dual‐Mode Ammonia Gas Detection. Adv. Funct. Mater. 2016, 26, 4329-4338. (7) Woo, H. S.; Na, C. W.; Kim, I. D.; Lee, J. H. Highly Sensitive and Selective Trimethylamine Sensor Using One-Dimensional ZnO-Cr2O3 Hetero-Nanostructures. Nanotechnology 2012, 23, 245501. (8) Chen, E. X.; Fu, H. R.; Lin, R.; Tan, Y. X.; Zhang, J. Highly Selective and Sensitive Trimethylamine Gas Sensor Based on Cobalt Imidazolate Framework Material. ACS Appl. Mater. Interfaces 2014, 6, 22871-22876. (9) Li, C.; Feng, C.; Qu, F.; Liu, J.; Zhu, L.; Lin, Y.; Wang, Y.; Li, F.; Zhou, J.; Ruan, S. Electrospun Nanofibers of p-type NiO/n-type ZnO Heterojunction with Different NiO Content and its Influence on Trimethylamine Sensing Properties. Sens. Actuators, B 2015, 207, 90-96. (10) Lee, S. H.; Lim, J. H.; Park, J.; Hong, S.; Park, T. H. Bioelectronic Nose Combined with a Microfluidic System for the Detection of Gaseous Trimethylamine. Biosens. Bioelectron. 2015, 71, 179-185. (11) Yang, S.; Liu, Y.; Chen, W.; Jin, W.; Zhou, J.; Zhang, H.; Zakharova, G. S. High Sensitivity


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