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Letter
Inter-nanofiber Spacing Adjustment in the Bundled Nanofibers for Sensitive Fluorescence Detection of Volatile Organic Compounds Zichao Zhou, Wei Xiong, Yifan Zhang, Dongjiang Yang, Tie Wang, Yanke Che, and Jincai Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00345 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017
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Analytical Chemistry
Inter-nanofiber Spacing Adjustment in the Bundled Nanofibers for Sensitive Fluorescence Detection of Volatile Organic Compounds Zichao Zhou,†,‡,§ Wei Xiong,†,‡,§ Yifan Zhang,†,‡ Dongjiang Yang,# Tie Wang,†,‡ Yanke Che,*,†,‡ and Jincai Zhao†,‡ †
Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. University of Chinese Academy of Sciences, Beijing 100049, China. # Department of Chemistry, Qingdao University, Qingdao 266071, China. *Corresponding author, Email:
[email protected]; Fax: +86-10-82617315. ‡
ABSTRACT: In this work, we report the fabrication of hierarchical nanofiber bundles from a perylene monoimide molecule that enable the sensitive detection of various inert VOCs. We demonstrate that the inter-nanofiber spacing of the bundles with appropriate packing interactions can be effectively adjusted by various VOCs, which is in turn translated into the dynamic fluorescence responses. Upon further decreasing the size of the nanofiber bundles, of which the inter-nanofiber spacing is more favorable to be adjusted, enhanced fluorescence responses to various VOC vapors can be achieved. Our work presents a new protocol, i.e., translating the stimuli-responsive inter-nanofiber spacing into fluorescence responses, to construct novel fluorescence sensors for various hazardous chemical vapors.
The development of low cost and portable sensors for volatile organic compounds (VOCs) has attracted intensive interest because of increasing concerns in security, environmental and industrial monitoring, and health.1-16 Among various detection technologies,2, 7-16 fluorescence sensors represent a costeffective and highly sensitive detection technology, which can also be miniaturized into portable or wearable devices.17 However, typical examples of sensitive fluorescence sensors are limited to those for some specific solvent18 or redox-active analytes, e.g., explosives17, 19-20 and amines.21-23 Most fluorescent sensors reported thus far had relatively low sensitivity to chemical inert VOCs, particularly at low concentrations. This is mainly because fluorescence responses to these VOCs arise from molecular packing changes of the sensing materials that requires either the elongated exposure to VOCs at moderate concentration24 or the exposure to VOCs at high concentration.12-13, 25-26 Therefore, the development of fluorescence sensors for sensitive detection of inert VOCs is highly desirable but requires the development of a novel sensing mechanism.
that allows the maximum adjustment of inter-nanofiber spacing to detect various VOCs through sensitive fluorescence responses. Of particular interest is that the hierarchical nanofiber bundles simply exhibited enhanced emission when exposed to most solvent vapors at a wide concentration window, while exhibited initially increased and then dropped emission upon exposure to ethanol and alkane vapors. Given that ethanol and alkane are the main components of gasoline, our sensor can thus be used to sensitively detect gasoline. To access optimized bundles that allow the maximum adjustment of the inter-nanofiber spacing by VOC vapors, molecules 1-5 that have a branched side chain of different length were synthesized for comparison (Figure 1a). Upon injection of a 0.5 mL solution of molecules 1-5 in chloroform (1 mM) into 5 mL hexane in a vial and aging for 30 min, the corresponding entangled nanofiber bundles were formed and suspended in solution. Scanning electron microscope (SEM) clearly showed the entangled morphology of the resulting nanofibers from 1-5 (Figure 1). Magnified SEM images further revealed that nanofibers from 1-3 are 40-60 nm in diameter, which bound each other into hierarchical nanofiber bundles (Insets in Figures 1b-d), while nanofibers from 4 and 5 seldom bound together but individually entangled (Insets in Figures 1e-f). This is probably because the weak hydrophobic interactions between shorter branched alkyl groups cannot glue nanofibers from 4 and 5 into the hierarchical bundles as nanofibers from 1-3 formed. Interestingly, the densely bundled nanofibers exhibited a higher fluorescence quantum yield (e.g., 17% over the nanofiber bundles from 1) than those relatively loosely packed ones (15%, 11%, 8%, and 5% over the nanofibers from 2-5, respectively). Given that the similar intra-nanofiber molecular arrangement as evidenced by their
Compared to perturbation of molecular packing within the sensing materials for the fluorescence response, adjustment of inter-aggregates spacing in hierarchical bundles by VOC molecules should have a smaller energetic barrier and thereby be accessible. Recently, densely and loosely bundled nanotubes have been reported to generate the distinct emission efficiency.27 This observation inspired us to explore the possibility of using the VOC-induced inter-nanofiber spacing variation in hierarchical bundles to realize fluorescence responses and thus enable sensitive detection of various VOCs. In this work, hierarchical nanofiber bundles are fabricated from perylene monoimide molecules, of which the bundled nanofibers from 3 (Figure 1a) have the appropriate inter-nanofiber binding force
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enhancement should arise from the annealing effect of VOC molecules that shortened the inter-nanofiber spacing for the inter-nanofiber exciton delocalization and thus enhanced the emission (Figure 2c). The exceptions are ethanol and alkane, which caused the fluorescence quenching of the nanofiber bundles from 3 at the whole concentration window (Figure 2b). This is because ethanol and alkanes have relatively strong interactions with the nanofibers that cause the inter-nanofiber swelling (the annealing effect is negligible for the bundles here) and thereby decreased emission (Figure 2c). Given that ethanol and alkanes are main components of gasoline, it is expected that these nanofiber bundles enable the detection of gasoline vapor based on similar fluorescence quenching responses. Indeed, marked fluorescence quenching responses were observed over the nanofiber bundles when blowing diluted gasoline vapor (Figure 2b). These observations suggest that such nanofiber bundles enabled the discrimination of certain VOCs against others based on distinct fluorescence response behaviors.
similar absorption and fluorescence spectra (Figure S1), the increased emission efficiency in densely bundled nanofibers should arise from the favorable inter-nanofiber exciton diffusion, as is the case in the nanotube bundles.27
Figure 1. (a) Molecular structures of compounds 1-5. (b-f) SEM images of nanofibers assembled from 1 (b), 2 (c), 3 (d), 4 (e), and 5 (f). Scale bar: 2 µm; scale bar of insets: 500 nm.
Having fabricated various nanofiber bundles, we next explored if external stimuli (i.e., VOCs) enable the adjustment of the inter-nanofiber spacing and generate dynamic emission changes, which can be used to develop a novel type of fluorescence sensors. We carried out the sensing experiments for various VOC vapors in a home-built optical chamber (Figure S2) coupled with an Ocean Optics USB4000 fluorometer. Upon exposure to various VOC vapors for ca. 1 s (Figure S3), the real-time fluorescence responses of the nanofiber bundles were recorded and compared. The nanofiber bundles from 1 exhibited very weak fluorescence responses to the tested VOCs, even at high concentrations (Figure S4). In contrast, the nanofiber bundles from 2 and 3 started to exhibit increased fluorescence responses to various VOCs under identical conditions (Figures S5 and 2). However, the entangled nanofibers from 5 and 6 again exhibited decreased responses to VOCs under identical conditions (Figures S6 and S7). These results can be explained by the interplay of the intrinsic internanofiber binding within nanofiber bundles and the intervention of the diffused VOC molecules in the bundle; the dynamic competitive interactions result in dynamic changes of the inter-nanofiber spacing and thereby the fluorescence responses. The nanofiber bundles from 1 have strong inter-nanofiber hydrophobic interactions and thus are difficult to be adjusted. By contrast, the entangled nanofibers from 4 and 5 seldom formed the nanofiber bundles and thereby can’t provide the necessary adjustment of inter-nanofiber spacing for fluorescence responses. Only the bundled nanofibers that have appropriate inter-nanofiber binding force (i.e., 3 nanofiber bundles) enabled the maximum adjustment of inter-nanofiber spacing and thus the maximum fluorescence responses upon exposure to VOCs (Figure S8). Notably, the nanofiber bundles from 3 exhibited emission enhancement when exposed to most VOC vapors at a wide concentration widow (Figure 2a) and emission enhancement-quenching responses upon exposure to very concentrated VOCs (Figure S9), respectively. Such emission
Figure 2. (a) Time-dependent fluorescence profile of the nanofiber bundles from 3 upon exposure to various VOCs at different concentrations. (b) Time-dependent fluorescence profile of nanofibers from 3 upon exposure to cyclohexane, ethanol, octane, and gasoline vapors, respectively. (c) Schematic diagram of different adjustment of the inter-nanofiber spacing by various VOCs.
The above observations motivated us to explore the possibility of further increasing the detection sensitivity of 3 nanofibers by manipulating their inter-nanofiber interactions. To this end, we developed a new self-assembling approach to achieve nanofiber bundles of a smaller size, the internanofiber force of which is expected to be weaker. Typically, a 1 mL chloroform solution of 3 (3 mM) was injected into 2 mL hexane in a vial and allowed to self-assemble for 30 min. The resulting solution containing suspended nanofibers along with considerable individual molecules was then drop-cast on
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Analytical Chemistry bundles are subject to both the annealing and swelling effects from these VOC molecules. This enhancement-quenching response behavior has an apparent advantage, which makes the discrimination of ethanol or alkane from water and amine vapors that can cause the fluorescence quenching of the nanofiber bundles (Figure S12).
a glass slide or in a tube to process further self-assembly where the individual molecules formed smaller nanofiber bundles on the pre-fabricated larger nanofibers with the solvent vaporization. SEM images confirmed the hierarchical nanofiber network (Figures 3 and S10), where the top network layer consisted of randomly entangled smaller nanofiber bundles formed by nanofibers with diameters of 40-60 nm (Figure 3b); the bottom layer consisted of densely packed larger bundles formed by pre-fabricated nanofibers with similar diameters (Figure S10). Notably, despite the different assembly methods, the resulting hierarchical nanofiber network showed the same optical spectra as those above (Figure S11), indicative of the same molecular organization within these assemblies. Interestingly, under the fluorescence image recorded with a confocal laser scanning microscopy (CLSM) (Figure 3d), the weaker emission (dark area) rather than stronger emission was observed in the relatively thick part of the nanofibril film, suggesting that the top and bottom layers of the hierarchical nanofiber network have distinct emission efficiency. This was corroborated by the measured fluorescence quantum yield of 8 % over the hierarchical nanofiber network, which is much lower than that of the nanofiber bundles pre-fabricated in solution (11%). These observations further indicate that compared to the smaller nanofiber bundles, the larger nanofiber bundles that are densely packed allow exciton to diffuse more efficiently28 and thereby give rise to a higher fluorescence quantum yield.
Figure 4. (a) Time-dependent fluorescence profile of the hierarchical nanofiber network from 3 upon exposure to various VOCs. (b) Time-dependent fluorescence profile of the hierarchical nanofiber network from 3 upon exposure to cyclohexane, ethanol, octane, and gasoline vapor, respectively. (c) The lowest detectable concentration of various VOCs over the hierarchical nanofiber network (1: 1,4-dioxane; 2: Tetrahydrofuran; 3: toluene; 4: butyl acetate; 5: chloroform; 6: acetone; 7: diethyl ether; 8: acetonitrile; 9: cyclohexane; 10: ethanol; 11: octane). (d) Comparison of fluorescence enhanced responses of the hierarchical nanofiber network (dark cyan) and large nanofiber bundles from 3 (gray) upon exposure to various VOCs 1-8 shown in (c) at the same concentration. Error bars represent the standard deviation of five measurements.
Figure 3. (a) SEM image of the hierarchical nanofiber network from 3 fabricated by a new self-assembly process. (b) Magnified SEM image of the top layer of the resulting hierarchical network. (c) Bright-field and (d) fluorescence CLSM image of the hierarchical nanofiber network from 3.
We envision that the hierarchical nanofiber network that contains relatively loosely packed smaller bundles would facilitate the adjustment of the inter-nanofiber spacing by VOCs, thereby enhancing the detection sensitivity. Indeed, marked emission enhancement of the hierarchical nanofiber network was observed compared to the mere nanofiber bundles above upon exposure to most VOC vapors at the same concentration (Figures 4a and d). The lowest detectable concentration of VOCs (we can test in our laboratory) was listed in Figure 4c. Among them, the lowest detectable concentration of diethyl ether was improved from ca. 1160 ppm to 116 ppm (Figure 4a). More interestingly, the hierarchical nanofiber network demonstrated initial fluorescence enhancement and then quenching upon exposure to ethanol, octane, and gasoline vapors (Figure 4b). The emerging fluorescence enhancement and quenching here are likely because the small nanofiber
To further corroborate that the above fluorescence responses to VOCs originate from the dynamic adjustment of the inter-nanofiber spacing, we monitored the fluorescence spectra changes of the hierarchical nanofiber network from 3 upon exposure to VOC vapors at elongated time. We performed such experiments by injecting the solvents at both terminals of the quartz tube whose middle interior surface was predeposited by the hierarchical nanofiber network and then allowing the solvent to vaporize and diffuse to the nanofiber network (Figure S13). A typical fluorescence change of the nanofiber network induced by a good solvent vapor (e.g., THF) was shown in Figure 5a, in which the fluorescence band remained unaltered but the intensity increased remarkably at the initial 2 min. The increased fluorescence intensity should arise from the reduced inter-nanofiber spacing rather than the
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Author Contributions
intra-nanofiber molecular reorganization because molecular reorganization within the nanofiber would generate the changes of the fluorescence band. Notably, with time progresses, the fluorescence band underwent a blue shift from 640 nm to 620 nm, accompanied by the decreased intensity (Figure 5b), indicative of the occurrence of intra-nanofiber molecular reorganization due to the mediation of THF molecules. For comparison, the influence of the fluorescence spectra of the nanofiber network by a poor solvent vapor (e.g., ethanol) was also investigated under the same conditions. Unlike the case of THF, the fluorescence intensity of the nanofiber network increased at the initial 1 min and then decreased slowly while the fluorescence band of the nanofiber network remained unaltered with time progressed (Figures 5c and 5d). Obviously, the poor solvent molecules cannot induce the intra-nanofiber molecular reorganization even upon elongated exposure to the vapor. They can only adjust the inter-nanofiber spacing via the competitive annealing and swelling effects toward dynamic fluorescence responses.
§
Z. Z and W. X contributed to this work equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by 973 project (No. 2013CB632405), NSFC (Nos. 21577147, 21590811, and 21521062), the “Strategic Priority Research Program” of the CAS (No. XDA09030200) and the “Youth 1000 Talent Plan” Fund.
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Figure 5. (a, b) Typical fluorescence changes of the hierarchical nanofiber network from 3 upon elongated exposure to THF vapor. (c, d) Typical fluorescence changes of the hierarchical nanofiber network from 3 upon elongated exposure to ethanol vapor.
In conclusion, the nanofiber bundles and hierarchical nanofiber network were assembled from perylene monoimide 3 to sensitively detect various VOCs through fluorescence responses. We demonstrate that the inter-nanofiber spacing of the nanofiber bundles with appropriate inter-nanofiber interaction force can be effectively adjusted by various VOCs, affording the apparent dynamic fluorescence responses. Upon further decreasing the size of the nanofiber bundles, of which the inter-nanofiber spacing is more favorable to be adjusted, enhanced fluorescence responses to various VOC vapors were achieved. Our work provides a new protocol, i.e., translating the stimuli-induced hierarchical inter-nanofiber spacing changes into fluorescence responses, to develop sensitive fluorescence sensors for hazardous chemical vapors.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic and characterization of molecules 1-5, sensing measurements, and other supporting figures and table (PDF)
AUTHOR INFORMATION Corresponding Author * Email:
[email protected].
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