Hollow-Core-Photonic-Crystal-Fiber-Based Miniaturized Sensor for

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Technical Note

A hollow core photonics crystal fiber based miniaturized sensor for detection of aggregation-induced emission molecules Huifang Chen, Qinjian Jiang, Yanqing Qiu, xuechun Chen, Bo Fan, Yi Wang, and Dongning Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03219 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on November 26, 2018

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Analytical Chemistry

A hollow core photonics crystal fiber based miniaturized sensor for detection of aggregation-induced emission molecules H.F. Chen,† § Q.J. Jiang,† § Y.Q. Qiu,† X.C. Chen,‡ B. Fan,‡ Y. Wang,*‡ D.N. Wang*† † College of Optical and Electronic Technology, China Jiliang University, Hangzhou, Zhejiang, China ‡ College of Pharmaceutical Sciences, Zhejiang University, Hangzhou, Zhejiang , China § State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiaotong University, Shanghai, China

ABSTRACT: A miniature sensor for detection of aggregation-induced emission (AIE) molecule is proposed in this work. The sensing head is fabricated by use of hollow core photonic crystal fiber with a core diameter of about 4.8 μm. The cladding holes are sealed by fusion splicing technique while the central hole remains open to allow the filtration of solution with AIE molecules. When the solution is excited by ultraviolet lamp, the fluorescence light is received by a fiber optic spectrometer. The fluorescence intensity is associated with the concentration of AIE molecules and the infiltrated core length. In the whole process of the experiments, the output peak wavelength is stable which indicates that the existing forms of AIE particles are stable and the fluorescence reabsorption can be neglected. The experimental results obtained are in accordance with the traditional microplate spectrophotometer methods. The most exciting result is the amount of sample measured can be as low as 0.36 nL, which allows detection of AIE molecules at only 0.02 pmol. In addition, the miniature sensor was successfully applied to detect an AIE-based bio-probe for evaluating the activity of dipeptidyl-peptidase 4 (DPP-4) inhibitor sitagliptin with IC50 of 59.80±3.06 nM. The advantage of small device size and nanoliter scale sample volume suggested the proposed sensor is promising in many biosensing applications.

F

luorescence detection is an important and powerful method in chemical, biomedical, and environmental researches. It has been an irreplaceable detection approach in many applications due to its superiority of strong selectivity and high sensitivity. However, there is an obstacle in traditional material detection due to aggregation-caused quenching (ACQ) phenomenon. In 2001, Tang et al.1 observed siloles molecules such as 1-Methyl-1,2,3,4,5-Pentaphenylsilole can emit strong fluorescence when they aggregate. This novel luminescence phenomenon is defined as the aggregation-induced emission (AIE) effect. In contrast to ACQ molecules, AIE molecules demonstrate strong light emission in high concentration solution, which makes AIE-based fluorophores attractive as a flexible design and easy synthesis can be achieved.2-5 So far, the detection of AIE materials are chiefly carried out by fluorescence spectrometer, microplate reader or microscope, which requests microliter to milliliter of samples to obtain satisfied sensitivity. Recently, fabrication of miniaturized sensors to facilitate low-cost and low-resources analysis gains great interest. Optical fiber fluorescence sensor has been widely employed because of its small size, low cost and intrinsic advantages of high signal-to-noise ratio, almost lossless transmission and biocompatibility, etc. The efficiency of collecting fluorescence signal is one of the most important factors in a detection system. Although fiber tip collector has simple structure,6-8 only a little fluorescence light can be captured by its small end facet. Nanofiber is another solution for collecting fluorescence light within its evanescent field.9-11 Its detection sensitivity can be enhanced by the relatively large coupling area; however, the

fragile structure limits its usage in practical applications. Microstructured optical fiber (MOF) has been taken as more effective fluorescence sensor in recent years.12-17 By infiltrating with fluorophores solution, the air holes inside can serve as the sample cells, delivery channels and light collectors with a long interaction path. Many kinds of MOF devices have been employed for fluorescence detection. The suspended core fiber18-21 and hollow-core photonics crystal fiber (HCPCF)22-24 with all air holes filled have been played the role as traditional fluorescent material (Rhodamine, for example) detector and only core filled HCPCF has also been reported in the recent research.25-27 To receive enough fluorescence light, the interaction path should be long, which results in the device length of dozens of centimeters. In this paper, an ultra-compact fiber device for AIE fluorophores detection is demonstrated. When the device is illuminated by an ultraviolet (UV) lamp, the fluorescence excited from AIE molecules of solution infiltrated in the core of HCPCF is coupled into the guide mode and transmitted to the spectrometer. The concentration of AIE molecules can be determined by the fluorescence intensity. The refractive index (RI) of the core filled with liquid is much higher than air-holes cladding, which allows total internal reflection for almost any fluorescence wavelength and hence supports multiple substances detection. The minimum length of fiber device is only 0.5 cm, which means the sample volume is down to 0.36 nL. This is the minimum sample volume of fiber fluorescence sensing systems as far as we know, indicating its great potential in biosensing and cell imaging applications. This sensor was successfully applied to determine excitation of two different

ACS Paragon Plus Environment

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

AIE molecules, Tetraphenylethylene-Lysine-PhenylalanineProline-Glutamic acid (TPE-KFPE) and 1-allyl-1-methyl2,3,4,5-tetraphenylsilole (AMTPS). Moreover, as a successful example, this miniaturized sensor exhibited excellent performance for in vitro detection of human dipeptidylpeptidase 4 (DPP-4) activities using TPE-KFPE, an AIE-based bio-probe, suggesting its vast potential for biomedical applications in clinical diagnosis and drug discovery for type 2 diabetes mellitus (T2DM).28

EXPERIMENTAL SECTION Samples and Device Preparation. TPE-COOH was purchased from Yu Lan Biotechnology Co., Ltd. (purity: 98%). AMTPS was provided by Dr. Cao29 from Zhejiang Sci-Tech University. And the chemical structures of these two compounds are shown in Figure S1. For TPE-COOH, samples and device preparation are described as follows. Firstly, 100 mM TPE-COOH solution was obtained by dissolving 20.3 mg of TPE-COOH powder in 0.5 mL of dimethyl sulphoxide (DMSO); secondly, by adding 9.5 mL of high pure water (ELGA Purelab) to the solution, the concentration was diluted down to 5 mM; finally, the solution of lower concentrations was obtained by dilution with high pure water. The refractive index of solution is ~1.3348, measured with an Abbe refractometer (@589.3 nm, 25 ℃ ). The photoluminescence (PL) spectra of AMTPS was measured in the same manner as TPE-COOH. The selective infiltrated fiber device is fabricated by use of fusion splicer technique.30 The fiber is commercial HCPCF (HC-532-02) provided by NKT PHOTONICS CO. It guides light in a hollow core, surrounded by a microstructured cladding formed by a periodic arrangement of air holes in silica. The fiber is designed for 530 nm center wavelength with a core diameter of ~4.8 μm, a microstructured region diameter of ~23 μm and a cladding diameter of ~81 μm (Figure 1(a)). When discharging on the end facet of the HCPCF, the central hole and cladding holes collapsed with different speeds due to different hole sizes. By selecting appropriate parameters of fusion splicing, the cladding holes could be sealed while the core hole remained to be open. The device with collapsed cladding holes was shown in Figure 1(b). By immersing the processed fiber end in TPE-COOH solution, the liquid would fill the core by capillary force. Under UV light illumination, the fluorophores were excited, and a great portion of fluorescence was coupled to the guide mode of the liquid core, while the others were scattered by the silica microstructure (Figure 1(c)). To testify if the core was the only filled hole, a short section of HCPCF with all air holes opened was filled with the same solution for comparison. In the UV source microscope image corresponding

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to all air holes filled device, the light spots could be observed in the whole microstructured region, and the device was much brighter than that with the only core filled (Figure 1(d)). Instrumental Setups. The schematic of experimental setup is shown in Figure 2. The core infiltrated fiber device with cleaved end had the length of ~2 cm, connected with a multi-mode fiber (MMF, 62.5 μm core diameter, 125 μm cladding diameter) by use of mechanical splice. A steady UV lamp, 3W of power, was used as the exciting source, with center wavelength of 372 nm and bandwidth of 20 nm. The fiber device was illuminated from upside. The AIE signal coming out from the AIE molecules was mostly coupled to the core mode, transmitted in MMF and received by a fiber optic spectrometer. Thus, the category of fluorophores could be determined from the wavelength of the received light, and the concentration of solution could be obtained from the intensity of signal. The fiber optic spectrometer is Jiuao JA2000, working at the visible spectrum. DPP-4 activity was detected by an AIE probe introduced by Wang et al.,31 with subtle changes. Briefly, TPE-KFPE, inhibitor (here sitagliptin) and DPP-4 were incubated in 37 ℃ for 30 minutes following detection of fluorescence intensity at 450 nm. Fluorescence signal was captured by our miniaturized device instead of microplate spectrophotometer. The detailed information of DPP-4 activity detection is given in Supporting Information.

Figure 2. Experimental setup for AIE molecules detection (inset: core filled HCPCF).

RESULTS AND DISCUSSION The first experiment was performed to study the relation between PL intensity and solution concentration. During the initial test, the solution of 5 mM concentration was infiltrated into the core of HCPCF. To speed up the infiltrating process, pressure differential method was used with an injector. When the solution in the core stop flowing, exposed the fiber device under the UV lamp and recorded the excited fluorescence light within guide modes by the spectrometer. Within the range of 400-525 nm, the PL intensity had single peak distribution centered at ~460 nm. Then the solution was diluted by use of pure water to 4.5 mM concentration. Next, drew the solution out of the core and flushed the hollow core with the diluted solution twice. The diluted solution was infiltrated to the core and the excited fluorescence intensity was recorded again. With these diluting-flushing-infiltrating processes, the PL intensity varied with different concentrations (Figure 3(a)). With the decrease of the solution concentration, less TPE-COOH

Figure 1. Schematic illustration of HCPCF device. (a) Cross view of HCPCF. (b) Micrograph of HCPCF end with collapsed ACS Paragon cladding holes. (c) Micrograph of device, core infiltrated with Plus Environment TPE-COOH solution, illuminated by UV lamp. (d) Micrograph of full infiltrated device.

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Analytical Chemistry molecules were contained in the core region, and the PL intensity was reduced. The relation between concentration and PL intensity is shown in Figure 3(b). In the lower concentration range, 0.05-3.5 mM, there is a sharp increase of PL intensity with the increase of solution concentration and a linear relationship exists. When the concentration was above 3.5 mM, the PL intensity changed only slightly, caused by approaching to the stabilization of AIE effect. The limit of detection is calculated to be 7.27 μM. These characters are quite similar with the results of TPE molecule obtained with traditional spectrophotometers.3,32,33 Experimental investigation on the excitation length of the device was carried out. The device infiltrated with 5 mM solution was applied. By use of a shelter with square hole at the center, the excitation length could be controlled at the connection end of the MMF. The spectra of 0.5-2 cm excitation lengths are shown in Figure 3(d). With a longer excitation length, there are more TPE-COOH molecules be excited, which result in a stronger PL intensity. The detected peak intensity has a linear relationship with the excitation length (Figure 3(e)).

as well, which implies that the fluorescence reabsorption introduced by transmission path can be neglected in the detecting system.27,35,36 The experimental results reveal that the HCPCF device can be flexibly designed and operated within a wide concentration range. The fabricated biosensor was also successfully utilized to detect AIE phenomena of another AIE molecule, AMTPS. A linear relationship between concentration and PL intensity of AMTPS is shown in 0.1-2 mM concentration range (Figure 4) with the detection limit being 1.48 μM. It indicates this sensor is capable in detection of AIE molecules with different structures. Moreover, to validate the capability of the proposed sensor in

Figure 4. PL spectra of AMTPS with different concentrations. (a) PL spectra evolution of AMTPS; (b) PL peak intensity. biomedical application, we detected the DPP-4 activity by using a previous reported AIE-based bio-probe TPE-KFPE.31 As shown in Figure 5(a), co-incubation with DPP-4 significantly increases the fluorescent signal of TPE-KFPE (from dark blue line to black line), indicating the formation of AIE molecules can be clearly detected. Moreover, when adding a known DPP-4 inhibitor sitagliptin with different concentration, the fluorescent intensities were decreased accordingly. The dose-dependent inhibition of sitagliptin on DPP-4 activity was calculated as shown in Figure 5(b). The IC50 of sitagliptin is 59.80±3.06 nM (calculated from 5 replicates), which is close to literature data that in vitro IC50 of sitagliptin on DPP-4 is around 40nM.37 Traditional microplate spectrophotometer requires sample

Figure 3. PL spectra of TPE-COOH with different concentrations and exciting length. (a) PL spectra of TPE-COOH with 2 cm excitation length. (b) PL peak intensity of different concentrations. (c) Peak wavelength with different concentrations. (d) PL spectra of TPE-COOH with 5 mM concentration. (e) PL Peak intensity with different excitation length. (f) Peak wavelength with different excitation lengths.

The optical property of the emission light has also been investigated. The PL peak wavelengths with different concentrations are demonstrated in Figure 3(c) and 3(f), extracted from the spectra shown in Figure 3(a). In the whole concentration range, the peak wavelength has only a small fluctuation, with the root-mean-square-error (RMSE) of 0.5599. The stable emission wavelength means that the existing forms of AIE particles in the mixtures did not change with the increase of solution concentration.2,34 The PL peak wavelengths with different excitation lengths are shown in Figure 3(f), extracted from the spectra Figure 3(d). The output wavelengths are stable

Figure 5. Application of miniaturized sensor for detection of DPP-4 activity. (a) PL spectra of AIE-based bioprobe with DPP-4. (b) The dose-dependent inhibition of sitagliptin on DPP-4 activity. liquid to overspread the bottom of microplate, which means the volume should be at least at microliter level even for 1536 wells microplate. Combined with precise picoliter dispenser instruments, our miniaturized device is a powerful detector for small sample in the field of clinical diagnosis and highthroughput drug screening.

CONCLUSION In conclusion, an ultra-compact, HCPCF based AIE molecule detecting device has been developed. The type of AIE molecule and the concentration of solution can be obtained from peak wavelength and peak intensity of PL spectrum respectively. The



output wavelength is stable, which proves that fluorescence reabsorption can be neglected and the existing forms of AIE particles do not change. The smallest size of our fiber device fabricated is 0.5 cm and the volume of solution sample achieved is only 0.36 nL. Such a compact structure and small sample volume make our device promising in many living cell imaging, drug screening and biosensing applications.

AUTHOR INFORMATION Corresponding Author * Yi Wang, College of Pharmaceutical Sciences, Zhejiang University (Zijingang Campus), Hangzhou, 310058, P. R. China, E-mail: [email protected]. * Dongning Wang, College of Optical and Electronic Technology, China Jiliang University, Hangzhou, 310018, P.R. China, E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 61661166009), the National Natural Science Foundation of China (Grant Nos. 81822047), the National Key Scientific and Technological Project of China (Grant Nos. 2017ZX09301012), and the State Key Laboratory of Advanced Optical Communication Systems and Networks.

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Analytical Chemistry

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Description for supplementary cover image The photoluminescence coupled into the transmission modes in the core of hollow core phonics crystal fiber, which can help to reduce the amount of sample required for AIE molecule detection.


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Hollow-Core-Photonic-Crystal-Fiber-Based Miniaturized Sensor for

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