Stimuli-Responsive Luminescent Copper Nanoclusters in Alginate

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Stimuli#Responsive Luminescent Copper Nanoclusters in Alginate and Their Sensing Ability for Glucose Siyu Gou, Yu-e Shi, Pan Li, Henggang Wang, Tianzi Li, Xuming Zhuang, Wei Li, and Zhenguang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20835 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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ACS Applied Materials & Interfaces

Stimuli‐Responsive Luminescent Copper Nanoclusters in Alginate and Their Sensing Ability for Glucose Siyu Gou,a Yu-e Shi, a,b,* Pan Li,a Henggang Wang, a Tianzi Li, a Xuming Zhuang,c,* Wei Li,d Zhenguang Wang a,b,* a

College of Chemistry and Environmental Science, Hebei University, Baoding, 071002,

P. R. China. Email: [email protected]; [email protected] b

Key Laboratory of Medicinal Chemistry and Molecular Diagnosis (Hebei University),

Ministry of Education, Baoding, 071002, P. R. China. c

College of Chemistry and Chemical Engineering, Yantai University, Yantai, 264005,

China. E-mail: [email protected] d School

of Pharmacy, Hebei University, Baoding, 071002, P. R. China.

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KEYWORDS: stimuli‐responsive materials, alginate, photoluminescence, metal nanoclusters, aggreagtion-induced emission, glucose

ABSTRACT: Visually observable pH-responsive luminescent materials are developed through integrating the properties of aggregation-induced emission enhancement of Cu nanocluster (NCs) and the Ca2+ triggered gelatin of alginate. Sodium alginate, CaCO3 nanoparticles and Cu NCs are dispersed in aqueous solution, which is in a transparent fluid state, showing a weak photoluminescence (PL). The introduced H+ can react with the CaCO3 nanoparticles to produce free Ca2+, which can cross-link the alginate chains into gel networks. Meanwhile, a dramatically increase on the PL intensity of Cu NCs and a blue shift on the PL peak appeared, assigned to the Ca2+ induced enhancement and gelatin induced enhancement, respectively. Their potential application as a sensor for glucose is also demonstrated based on the principle that glucose oxidase can recognize glucose and produce H+, which further triggers the above mentioned two-stage enhancement. A linear relationship between the PL intensity and concentration of glucose in the range of 0.1 to 2.0 mM is obtained, with a limit of detection calculated as 3.2×10-5 M.

1. Introduction Stimuli-responsive luminescent materials have gained much attention for the applications in the fields of optical devices, luminescent switches and chemical or biochemical sensors.1-4 Up to


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now, luminescent materials responding to external stimulus, such as humidity,5 temperature,6 pressure7 and pH8,9, have been developed. Among them, pH-responsive materials are becoming more and more important for the investigation of medicine, biology and environment issues.9-11 For example, pH is a fundamental feature for the synthesis of adenosine triphosphate (ATP), driven by a proton gradient. Moreover, pH-responsive luminescent materials can be conveniently integrated with other systems to fabricate multifunctional materials, which can further widen their scopes of use. In a typical pH-responsive luminescent system, the key component is luminescent materials, with preferred features of bright luminescence, low toxic and easy of synthesis. Metal nanoclusters (NCs) have emerged as such materials, due to their chemical and optical properties, such as strong photoluminescence (PL), good biocompatibility and well controlled solution synthesis.12-17 Some pH-responsive metal NCs have been reported, which can tune the PL properties driven under the mechanisms of deprotonation/protonation of chemical groups,18,19 aggregation-induced emission (AIE),20 electrostatic interactions21 and host-guest interactions22. Most of the above metal NCs show only PL response to the change of pH. With this single response, however, the applications of materials are limited. Therefore, it’s highly desirable to integrate additional response, a visually observable response for example, by artful management of used materials. Hydrogels are jelly-like materials synthesized by self-assembly of small molecules into gel, which have been widely used in the fabrication of biomedicine, electronic devices and chemical sensors.23-26 Gel formation is a straight-forward end-point measurement method, which can be detected visually without the use of complicated instrumentations. Among the reported raw materials for synthesis of hydrogels, alginate is a well-known matrix, due to its features of nontoxic, biocompatible and fast gel formation.27-29 Alginate contains chains of alternating α-Lguluronic acid (G) and β-D-mannuronic acid (M) residues, which can be obtained through extracting from seaweed. It can produce ionic cross-links by interacting with Ca2+, forming an “egg-box” structure, which is an ideal candidate for fabrication of visually observable pHresponse luminescent materials. In this work, visually observable pH-responsive luminescent materials are fabricated by dispersing Cu NCs into alginate solution, together with CaCO3 nanoparticles. After the introducing of H+, free Ca2+ is released from the CaCO3 nanoparticles, which further leads to the


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occur of inter-chain cross-liking and results in the formation of hydrogels. Simultaneously, an increase on the PL intensity of Cu NCs is observed, attributed to the AIE properties of Cu NCs, which can be explained by the Ca2+ induced enhancement and gelatin induced enhancement process. Its potential applications are demonstrated by developing as a chemical sensor for glucose, after integrating the pH-response properties with the ability to produce H+ in the reaction of glucose oxidase (GOx) with glucose.

2. Experimental Section 2.1. Materials. Glutathione (GSH) and copper nitrates (Cu (NO3)2) were purchased from Aladdin. D-Glucose, sodium alginate, L(+)-ascorbic acid (AA) and L-cysteine (Cys) bydrochloride anhydrous, calcium chloride (CaCl2) were purchased from Kermel (Tianjin, China). GOx was purchased from Sigma-Aldrich. CaCO3 nanoparticles, with diameter of 80 nm labeled, was obtained from Ruicheng NanoMaterials Technology Co.,Ltd. PBS buffer (0.01 M) was obtained from biosharp (Hefei,China). Galactose, fructose and dopamine hydrochloride were obtained from Beijing Innochem technology co., LTD. 2.2. Instruments. Transmission electron microscopy (TEM) images were acquired form a Philips CM 20 microscope operating at 200 kV. Fluorescence spectrums were obtained using SpectrofluorometerFS5. Scanning electron microscopy (SEM) images were obtained on an environmental scanning electron microscope (ESEM, FEI/Philips XL30). X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB-MKII 250 instrument (Thermo, USA). 2.3. Synthesis of Cu NCs. Typically, 2.0 mL of Cu(NO3)2 aqueous solution ( 50 mM) was injected into 18 mL of GSH solution (28 mM), under vigorous stirring. Then the pH of mixture was adjusted to 7.0 using NaOH solution (2 M), which became milky white and aqua in the end. A weak red emission can be observed under ultraviolet lamp irradiation. 2.4. Fabrication of pH-responsive luminescent materials. Alginate solutions were prepared by dissolving sodium alginate powder and keep for stirring for 3 h. Then, CaCO3 nanoparticles were dispersed into above solution under ultrasonication for 30 min. Finally, the synthesized Cu NCs were mixed with above solution with a volume ratio of 2:1, with a total volume of 5.0 mL. The


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as-obtained sample was in a transparent fluid state, showing a weak PL under ultraviolet lamp irradiation. After adding 0.1 mL of HCl solution (1.0 M), it transferred to free standing gel with a huge enhancement on the PL intensity. 2.5. Quantitative Detection of Glucose. A series of samples were prepared by mixing 3.33 ml of Cu NCs, 1.67 ml of alginate solutions containing GOx (10 U/mL), with different concentration of glucose (in the range from 0.1 to 5 mM) added, which were allowed to incubate under room temperature for 210 min. The PL spectra were finally recorded on spectrometers. 2.6 Selectivity and spike recovery test of proposed method. Dopamine, AA, fructose, lactose, Cys were selected as interferes to test the selectivity of as-proposed method to glucose. All the processes were totally same with the detection of glucose, excepting that the glucose was replaced with above chemicals, with a concentration of 2.0 mM. After a routine pretreating process of the whole blood, the 100-fold diluted samples were used as buffer for the detection of glucose. Then, proper amount of glucose solution was added to keep the final glucose concentrations as 0.5, 1.0, 2.0 mM, respectively. All other conditions were same with above measurements and the concentration of detected glucose were calculated according to the standard curve.

3. Result and Discussions 3.1. Characterization of Cu NCs. Cu NCs are synthesized by using GSH as both reduction reagents and ligands to stabilize the as-synthesized clusters. The morphology of Cu NCs is studied by TEM observations. As shown in Figure 1a, the as-synthesized Cu NCs are approximately 1.5 nm in diameter with a homogeneous distribution. The surface composition of Cu NCs is analyzed by XPS, as shown in Figure 1b and 1c. All the expected elements, including C, N, O, S and Cu are detected, with a ratio Cu to S of 1:4 calculated. To further study the valance state of Cu, high resolution XPS analysis was conducted, as shown in Figure 1c. The full


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reduction of Cu2+ is confirmed by the absence of peaks at 942 eV, while peaks around 932 and 952 eV correspond to Cu 2p3/2 and Cu 2p1/2 states of Cu(0)/Cu(I) in the ligand-stabilized Cu NCs, consistent with previously reported works.14,15 The as-synthesized Cu NCs show a weak red emission under the radiation of UV light, whose PL and PL excitation (PLE) spectra are peaked at 625 and 358 nm, respectively (Figure 1d). To further study the composition of Cu NCs and its relationships with luminescence properties, MALDI-TOF-MS spectrum of Cu NCs was measured. As shown in Figure S1, the major peaks at m/z=1358.1 and 1175.66 can be attributed to the formula of Cu2L4 and Cu4L3, respectively (where L = C10H16O6N3S). Other peaks at m/z=985.41, 917.57, 741.08 and 678.0 are ascribed to the fragments of CuL3, (L-H)3, Cu2L2, CuL2, respectively. Estimating form the composition and intensity, Cu2 and Cu4 might be the prominent components in the Cu NCs. The size of Cu NCs can be attributed to the synergistic effects of ligands and metal core. The difference of above two components is adding 2 Cu atoms and deleting 1 L, which will not lead to obvious changes on the final size. The composition of Cu NCs is also consistent with their PL properties and previous reports,13,14 where Cu NCs stabilized by strong bound ligands or small sized cluster (

Stimuli-Responsive Luminescent Copper Nanoclusters in Alginate

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